Multicore fiber with exterior cladding region

ABSTRACT

A multicore optical fiber includes two or more cores, a common interior cladding surrounding the two or more cores, and a common exterior cladding surrounding the common interior cladding. The common exterior cladding has a lower relative refractive index than the common interior cladding and reduces tunneling losses from the cores. The reduced tunneling loss allows placement of cores closer to the edge of the fiber, thus providing multicore optical fibers having higher core count for a given fiber diameter. Separation between cores is controlled to minimize crosstalk.

This application claims the benefit of priority from U.S. ProvisionalPatent Application Ser. No. 62/991,898 filed on Mar. 19, 2020, thecontent of which is relied upon and incorporated herein by reference inits entirety.

FIELD OF THE DISCLOSURE

This disclosure pertains to optical fibers. More particularly, thisdisclosure relates to multicore optical fibers. Most particularly, thisdisclosure relates to multicore optical fibers having high core density.

BACKGROUND OF THE DISCLOSURE

Optical fibers are utilized in a variety of telecommunicationapplications. The most widely used optical fibers include a single coreregion for transmission of optical signals. The core region typicallyhas a diameter on the order of 10 μm and is surrounded by a claddinghaving a diameter of 125 μm. Since the transmission capacity ofsingle-core optical fibers is currently approaching its theoreticallimits, the demand for increased transmission capacity is currentlybeing met through increases in the number of single-core optical fibersincluded in transmission cables. While a higher fiber count provideshigher transmission capacity, it leads to larger cables and makes itdifficult to retrofit existing fiber installations, which areconstrained by space limitations, with higher capacity cables. As aresult, there is a need to develop solutions that provide highertransmission capacity without increasing the size of transmissioncables.

One solution under consideration is multicore optical fibers. Multicoreoptical fibers include multiple cores embedded in a single commoncladding matrix. Each core of a multicore optical fiber acts as anindependent transmission channel. Since transmission capacity increasesas the number of cores increases, it is desirable to maximize thedensity of cores in a given cross-sectional area of cladding. Coredensity can be increased by reducing the spacing between cores. As corespacing is reduced, however, crosstalk between cores occurs and signalquality degrades as signals transmitted in different cores mix. Coredensity is also limited by the spacing between cores and the outersurface of the cladding. The cladding is typically surrounded by one ormore protective coatings and signal intensity in cores proximate to theouter surface of the cladding is decreased by tunneling through thecladding into a protective coating. There is a need for multicore fibershaving high core density and minimal signal degradation.

SUMMARY

The present disclosure provides multicore optical fibers having highcore density. The multicore optical fiber includes a multicore glassfiber that includes two or more cores surrounded by a common claddingregion and an exterior cladding region that surrounds the commoncladding region. The exterior cladding region has a lower index than thecommon cladding region and suppresses tunneling of optical signals fromcores proximate to the outer surface of the multicore glass fiber.

The present description extends to:

A multicore optical fiber comprising:

-   -   a multicore glass fiber, the multicore glass fiber comprising:        -   a plurality of core regions, each of the plurality of core            regions surrounded by and directly adjacent to a dedicated            inner cladding region;        -   a common interior cladding region surrounding the dedicated            inner cladding region of each of the plurality of core            regions, the common interior cladding region having a            relative refractive index Δ₄ and a radius R₄; and    -   a common exterior cladding region surrounding the common        interior cladding region, the common exterior cladding region        having a radius R₅>R₄ and a relative refractive index Δ₅<Δ₄.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from the description or recognized by practicing theembodiments as described in the written description and claims hereof,as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary, and areintended to provide an overview or framework to understand the natureand character of the claims.

The accompanying drawings are included to provide a furtherunderstanding, and are incorporated in and constitute a part of thisspecification. The drawings are illustrative of selected aspects of thepresent disclosure, and together with the description serve to explainprinciples and operation of methods, products, and compositions embracedby the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a cross-section of a multicore glass fiber having twocore regions, each of which includes a dedicated inner cladding region,a dedicated depressed index cladding region, a common interior claddingregion, and a common exterior cladding region.

FIG. 2 depicts an exemplary step-index profile for a core in themulticore glass fiber shown in FIG. 1.

FIG. 3 depicts an exemplary α-profile for a core in the multicore glassfiber shown in FIG. 1.

FIG. 4 depicts a cross-section of a multicore glass fiber having twocore regions, each of which has a dedicated inner cladding region, acommon interior cladding region, and a common exterior cladding region.

FIG. 5 depicts an exemplary step-index profile for a core in themulticore glass fiber shown in FIG. 4.

FIG. 6 depicts an exemplary α-profile for a core in the multicore glassfiber shown in FIG. 4.

FIG. 7 depicts a cross-section of a multicore glass fiber having twocore regions, a common interior cladding region, and a common exteriorcladding region.

FIG. 8 depicts an exemplary step-index profile for a core in themulticore glass fiber shown in FIG. 7.

FIG. 9 depicts an exemplary step-index profile for a core in themulticore glass fiber shown in FIG. 7.

FIG. 10 depicts crosstalk in a multicore glass fiber.

FIG. 11 depicts tunneling loss at a wavelength of 1550 nm for a coreregion in a multicore optical fiber with and without a common exteriorcladding.

FIG. 12 depicts tunneling loss at a wavelength of 1550 nm for a coreregion in a multicore optical fiber with and without a common exteriorcladding.

FIGS. 13-17 show illustrative designs of multicore optical fibers.

DETAILED DESCRIPTION

The present disclosure is provided as an enabling teaching and can beunderstood more readily by reference to the following description,drawings, examples, and claims. To this end, those skilled in therelevant art will recognize and appreciate that many changes can be madeto the various aspects of the embodiments described herein, while stillobtaining the beneficial results. It will also be apparent that some ofthe desired benefits of the present embodiments can be obtained byselecting some of the features without utilizing other features.Accordingly, those who work in the art will recognize that manymodifications and adaptations are possible and can even be desirable incertain circumstances and are a part of the present disclosure.Therefore, it is to be understood that this disclosure is not limited tothe specific compositions, articles, devices, and methods disclosedunless otherwise specified. It is also to be understood that theterminology used herein is for the purposes of describing particularaspects only and is not intended to be limiting.

In this specification and in the claims that follow, reference will bemade to a number of terms which shall be defined to have the followingmeanings:

“Include,” “includes,” “including”, or like terms means encompassing butnot limited to, that is, inclusive and not exclusive.

As used herein, the term “about” means that amounts, sizes,formulations, parameters, and other quantities and characteristics arenot and need not be exact, but may be approximate and/or larger orsmaller, as desired, reflecting tolerances, conversion factors, roundingoff, measurement error and the like, and other factors known to those ofskill in the art. When a value is said to be about or about equal to acertain number, the value is within ±10% of the number. For example, avalue that is about 10 refers to a value between 9 and 11, inclusive.When the term “about” is used in describing a value or an end-point of arange, the disclosure should be understood to include the specific valueor end-point referred to. Whether or not a numerical value or end-pointof a range in the specification recites “about,” the numerical value orend-point of a range is intended to include two embodiments: onemodified by “about,” and one not modified by “about.” It will be furtherunderstood that the end-points of each of the ranges are significantboth in relation to the other end-point, and independently of the otherend-point.

The term “about” further references all terms in the range unlessotherwise stated. For example, about 1, 2, or 3 is equivalent to about1, about 2, or about 3, and further comprises from about 1-3, from about1-2, and from about 2-3. Specific and preferred values disclosed forcompositions, components, ingredients, additives, and like aspects, andranges thereof, are for illustration only; they do not exclude otherdefined values or other values within defined ranges. The compositionsand methods of the disclosure include those having any value or anycombination of the values, specific values, more specific values, andpreferred values described herein.

The indefinite article “a” or “an” and its corresponding definitearticle “the” as used herein means at least one, or one or more, unlessspecified otherwise. The term “plurality” means two or more.

Directional terms as used herein—for example up, down, right, left,front, back, top, bottom—are made only with reference to the figures asdrawn and the coordinate axis provided therewith and are not intended toimply absolute orientation.

As used herein, contact refers to direct contact or indirect contact.Direct contact refers to contact in the absence of an interveningmaterial and indirect contact refers to contact through one or moreintervening materials. Elements in direct contact touch each other.Elements in indirect contact do not touch each other, but do touch anintervening material or series of intervening materials, where theintervening material or at least one of the series of interveningmaterials touches the other element. Elements in contact may be rigidlyor non-rigidly joined. Contacting refers to placing two elements indirect or indirect contact. Elements in direct (indirect) contact may besaid to directly (indirectly) contact each other.

As used herein, “directly adjacent” means directly contacting and“indirectly adjacent” mean indirectly contacting. The term “adjacent”encompasses elements that are directly or indirectly adjacent to eachother.

“Optical fiber” refers to a waveguide having a glass portion surroundedby a coating. The glass portion includes a core and a cladding, and isreferred to herein as a “glass fiber”. A multicore optical fiber is anoptical fiber with a glass fiber that includes two or more coressurrounded by a cladding common to the two or more cores. Each core ofthe glass fiber of a multicore optical fiber functions as a waveguide.

“Radial position”, “radius”, or the radial coordinate “r” refers toradial position relative to the centerline (r=0) of a core of themulticore optical fiber. Each of the two or more cores of a multicoreoptical fiber has a centerline and a separate radial coordinate r.“Radial position”, “radius”, or the radial coordinate “R” refers toradial position relative to the centerline (R=0) of the multicoreoptical fiber. The multicore optical fiber has a single centerline. Theradial coordinate r will be used to refer to radial position in the coreregion and any of the dedicated cladding regions described herein. Theradial coordinate R will be used to refer to radial position in thecommon cladding region and the exterior cladding region describedherein.

“Refractive index” refers to the refractive index at a wavelength of1550 nm.

The “refractive index profile” is the relationship between refractiveindex or relative refractive index and radius. For relative refractiveindex profiles depicted herein as having step boundaries betweenadjacent core and/or cladding regions, normal variations in processingconditions may preclude obtaining sharp step boundaries at the interfaceof adjacent regions. It is to be understood that although boundaries ofrefractive index profiles may be depicted herein as step changes inrefractive index, the boundaries in practice may be rounded or otherwisedeviate from perfect step function characteristics. It is furtherunderstood that the value of the relative refractive index may vary withradial position r or R within a core region and/or any of the claddingregions. When relative refractive index varies with radial position r orR in a particular region of the fiber (e.g. core region and/or any ofthe dedicated, common, or exterior cladding regions described below), itis expressed in terms of its actual or approximate functionaldependence, or in terms of its value at a particular radial position ror R within the region, or in terms of an average value applicable tothe region as a whole. Unless otherwise specified, if the relativerefractive index of a region (e.g. core region and/or any of thecladding regions) is expressed as a single value or as a parameter (e.g.Δ or Δ%) applicable to the region as a whole, it is understood that therelative refractive index in the region is constant, or approximatelyconstant, and corresponds to the single value, or that the single valueor parameter represents an average value of a non-constant relativerefractive index dependence with radial position r or R in the region.For example, if i is a region of the glass fiber, the parameter Δ_(i)refers to the average value of relative refractive index in the region,unless otherwise specified. Whether by design or a consequence of normalmanufacturing variability, the dependence of relative refractive indexon radial position may be sloped, curved, or otherwise non-constant.

“Relative refractive index,” as used herein, is defined in Eq. (1) as:

$\begin{matrix}{{{\Delta(r)}\;\%} = {100\frac{\left( {{n^{2}(r)} - n_{ref}^{2}} \right)}{2{n^{2}(r)}}}} & (1)\end{matrix}$where n(r) is the refractive index at radial position r in the glassfiber, unless otherwise specified, and n_(ref) is the refractive indexof pure silica glass, unless otherwise specified. For purposes of thepresent disclosure, n_(ref)=1.444, which is the refractive index of puresilica at 1550 nm. Accordingly, as used herein, the relative refractiveindex percent is relative to pure silica glass. As used herein, therelative refractive index is represented by Δ (or “delta”) or Δ % (or“delta %) and its values are given in units of “%”, unless otherwisespecified. Relative refractive index may also be expressed as Δ(r) orΔ(r) %. An analogous definition of relative refractive index can beexpressed in terms of radial coordinate R.

The average relative refractive index (Δ_(ave)) of a region of the fiberis determined from Eq. (2):

$\begin{matrix}{\Delta_{ave} = {\int_{r_{inner}}^{r_{outer}}\frac{\Delta\;(r){dr}}{\left( {r_{outer} - r_{inner}} \right)}}} & (2)\end{matrix}$where r_(inner) is the inner radius of the region, r_(outer) is theouter radius of the region, and Δ(r) is the relative refractive index ofthe region. An analogous definition of average refractive index can beexpressed in terms of radial coordinate R.

The term “α-profile” or “alpha profile” refers to a relative refractiveindex profile Δ(r) that has the functional form defined in Eq. (3):

$\begin{matrix}{{\Delta(r)} = {{\Delta\left( r_{0} \right)}\left\lbrack {1 - \left\lbrack \frac{\left| {r - r_{0}} \right|}{\left( {r_{Z} - r_{0}} \right)} \right\rbrack^{\alpha}} \right\rbrack}} & (3)\end{matrix}$where r_(o) is the radial position at which Δ(r) is maximum, r_(z)>r₀ isthe radial position at which Δ(r) decreases to its minimum value, and ris in the range r_(i)≤r≤r_(f), where r_(i) is the initial radialposition of the α-profile, r_(f) is the final radial position of theα-profile, and α is a real number. An α-profile with an α value of 10 ormore is an example of a step-index profile. An α-profile with an α valueless than 10 is an example of a graded-index profile. Δ(r₀) for anα-profile may be referred to herein as Δ_(max) or, when referring to aspecific region i of the fiber, as Δ_(i,max). When the relativerefractive index profile of the fiber core region is described by anα-profile with r₀ occurring at the centerline (r=0) and r_(z)corresponding to the outer radius r₁ of the core region, Eq. (3)simplifies to Eq. (4):

$\begin{matrix}{{\Delta_{1}(r)} = {\Delta_{1\max}\left\lbrack {1 - \left\lbrack \frac{r}{r_{1}} \right\rbrack^{\alpha}} \right\rbrack}} & (4)\end{matrix}$

The “mode field diameter” or “MFD” of an optical fiber is defined in Eq.(5) as:

$\begin{matrix}{{{MFD} = {2w}}{w^{2} = {2\frac{\int_{0}^{\infty}{\left( {f(r)} \right)^{2}rdr}}{\int_{0}^{\infty}{\left( \frac{d{f(r)}}{dr} \right)^{2}rdr}}}}} & (5)\end{matrix}$where f(r) is the transverse component of the electric fielddistribution of the guided optical signal and r is radial position inthe fiber. “Mode field diameter” or “MFD” depends on the wavelength ofthe optical signal and is reported herein for a wavelength of 1550 nm.Specific indication of the wavelength will be made when referring tomode field diameter herein. Unless otherwise specified, mode fielddiameter refers to the LP₀₁ mode at the specified wavelength.

“Effective area” of an optical fiber is defined as:

$\begin{matrix}{A_{eff} = \frac{2{\pi\left\lbrack {\int_{0}^{\infty}{\left( {f(r)} \right)^{2}rdr}} \right\rbrack}^{2}}{\int_{0}^{\infty}{\left( {f(r)} \right)^{4}rdr}}} & (6)\end{matrix}$where f(r) is the transverse component of the electric field of theguided optical signal and r is radial position in the fiber. “Effectivearea” or “A_(eff)” depends on the wavelength of the optical signal andis understood herein to refer to a wavelength of 1550 nm.

Reference to a difference between a first quantity and a second quantitymeans the result obtained by subtracting the second quantity from thefirst quantity. For example, a difference between a radius r_(i) and aradius r_(j) refers to r_(i)−r_(j). Reference to a magnitude of aquantity or a magnitude of a difference refers to the absolute value ofthe quantity or the difference.

The cutoff wavelength of an optical fiber is the minimum wavelength atwhich the optical fiber will support only one propagating mode. Forwavelengths below the cutoff wavelength, multimode transmission mayoccur and an additional source of dispersion may arise to limit thefiber's information carrying capacity. Cutoff wavelength will bereported herein as a fiber cutoff wavelength or a cable cutoffwavelength. The fiber cutoff wavelength is based on a 2-meter fiberlength and the cable cutoff wavelength is based on a 22-meter cabledfiber length. The 22-meter cable cutoff wavelength is typically lessthan the 2-meter cutoff wavelength due to higher levels of bending andmechanical pressure in the cable environment. The fiber cutoffwavelength λ_(CF) is based on a 2-meter fiber length while the cablecutoff wavelength λ_(CC) is based on a 22-meter cabled fiber length asspecified in TIA-455-80: FOTP-80 IEC-60793-1-44 Optical Fibres—Part1-44: Measurement Methods and Test Procedures—Cut-off Wavelength (21 May2003), by Telecommunications Industry Association (TIA). Each core of amulticore optical fibers exhibits a cutoff wavelength. The cutoffwavelength may be the same or different for different cores.

“Chromatic dispersion”, herein referred to as “dispersion” unlessotherwise noted, of an optical fiber is the sum of the materialdispersion and the waveguide dispersion. The zero-dispersion wavelength(λ₀) is the wavelength at which the dispersion has a value of zero.Dispersion slope is the rate of change of dispersion with respect towavelength. Dispersion and dispersion slope are reported herein at awavelength of 1550 nm and are expressed in units of ps/nm·km andps/nm²·km, respectively. Each core of a multicore optical fibersexhibits dispersion. The dispersion may be the same or different fordifferent cores.

The multicore optical fibers disclosed herein include two or more coreregions, a cladding region that includes an interior cladding regioncommon to at least two of the two or more core regions, and an exteriorcladding region surrounding and directly adjacent to the interiorcladding region. The core regions and cladding regions are glass. Thecladding region includes multiple regions that differ in relativerefractive index. Types of cladding regions include dedicated claddingregions and common cladding regions. A cladding region is said to be“dedicated” if it surrounds only one core of the two or more cores andis said to be “common” if it surrounds at least two cores of the two ormore cores. In embodiments described herein, at least two of thecladding regions are common to two or more cores of the multicore fiber.Preferably, the at least two common cladding regions are common to allcores of the multicore optical fiber. Each core of the multicore opticalfiber includes at least one dedicated cladding region and at least twocommon cladding regions. In a preferred embodiment, each core regionincludes at least one dedicated cladding region directly adjacent to thecore region and in embodiments with two or more dedicated claddingregions, at least one of the two or more dedicated cladding regions isdirectly adjacent to the core region and each of the others of the twoor more dedicated cladding regions is directly adjacent to another ofthe two or more dedicated cladding region. In another preferredembodiment, one of the at least two common cladding regions is directlyadjacent to the dedicated cladding region furthest removed from the coreregion. A common cladding region directly adjacent to the core region orthe furthest removed of a series of one or more dedicated claddingregions is referred to herein as an interior common cladding region. Theoutermost (most radially distant) common cladding region is referred toherein as an exterior common cladding region. An exterior commoncladding region defines the outer surface of the glass fiber. In someembodiments, the exterior common cladding region surrounds and isdirectly adjacent to the interior common cladding region. In otherembodiments, one or more intermediate common cladding regions isdisposed between the interior common cladding region and the exteriorcommon cladding region. The multicore optical fibers preferably furtherinclude a coating surrounding the exterior cladding region.

In some embodiments, the cladding region of a core region includes adedicated inner cladding region surrounding and directly adjacent to thecore region, an interior common cladding region surrounding and directlyadjacent to the dedicated inner cladding region, and an exterior commoncladding region surrounding and directly adjacent to the interior commoncladding region. The relative refractive index of the dedicated innercladding region is less than the relative refractive index of the coreregion. The relative refractive index of the exterior common claddingregion is less than the relative refractive index of the interior commoncladding region. In some embodiments, the relative refractive index ofthe interior common cladding region is greater than the relativerefractive index of the exterior common cladding region and the relativerefractive index of the dedicated inner cladding region.

In some embodiments, the cladding region of a core region includes adedicated inner cladding region surrounding and directly adjacent to thecore region, a dedicated depressed index cladding region surrounding anddirectly adjacent to the dedicated inner cladding region, an interiorcommon cladding region surrounding and directly adjacent to thededicated depressed index cladding region, and an exterior commoncladding region surrounding and directly adjacent to the interior commoncladding region. The relative refractive index of the dedicated innercladding region is less than the relative refractive index of the coreregion and the relative refractive index of the dedicated depressedindex cladding region is less than the relative refractive index of thededicated inner cladding region. The relative refractive index of theexterior common cladding region is less than the relative refractiveindex of the interior common cladding region. In some embodiments, therelative refractive index of the exterior common cladding region is lessthan the relative refractive index of the interior common claddingregion and the relative refractive index of the dedicated inner claddingregion. In some embodiments, the relative refractive index of theinterior common cladding region is greater than the relative refractiveindex of the exterior common cladding region and the relative refractiveindex of the dedicated depressed index cladding region.

In some embodiments, the cladding region of a core region includes adedicated outer cladding region disposed between a dedicated innercladding region and an interior common cladding region or between adedicated depressed index cladding region and an interior commoncladding region. Preferably, when present, a dedicated outer claddingregion is surrounded by and directly adjacent to the interior commoncladding region and the dedicated outer cladding region surrounds and isdirectly adjacent to a dedicated inner cladding region or a dedicateddepressed index cladding region.

The core region, inner cladding region, depressed index cladding region,outer cladding region interior common cladding region and exteriorcommon cladding region are also referred to herein as core, cladding,inner cladding, depressed index cladding, outer cladding, interiorcladding and exterior cladding, respectively.

Whenever used herein, radial position r₁ and relative refractive indexΔ₁ or Δ₁(r) refer to a core region, radial position r₂ and relativerefractive index Δ₂ or Δ₂(r) refer to a dedicated inner cladding region,radial position r₃ and relative refractive index Δ₃ or Δ₃(r) refer to adedicated depressed index cladding region, radial position r₄ andrelative refractive index Δ_(4doc) or Δ_(4doc)(r) refer to a dedicatedouter cladding region, radial position R₄ and relative refractive indexΔ₄ or Δ₄(R) refer to an interior common cladding region, radial positionR₅ and relative refractive index Δ₅ or Δ₅(R) refer to an exterior commoncladding region, radial position R₆ refers to a primary coating, andradial position R₇ refers to a secondary coating. Each radial positionr_(i) (i=1, 2, 3, or 4) and R_(i) (i=4, 5, 6, or 7) refers to the outerradius of the region associated with the value i. For example, r₁ refersto the outer radius of a core region, r₂ refers to the outer radius of adedicated inner cladding region etc.

When helpful for purposes of clarity to identify radial positions andrelative refractive indices of different core regions, differentdedicated inner cladding regions, different dedicated depressed indexcladding regions, and/or different dedicated outer cladding regions ofthe multicore glass fiber, a second identifying subscript will be used.For example, the radius r_(ij) refers to the radial position r_(i) ofthe j^(th) region of type i in the multicore glass fiber and Δ_(i,j)refers to the relative refractive index Δ_(i) of the j^(th) region oftype i in the multicore glass fiber. Regions of type i include a coreregion (i=1), a dedicated inner cladding region (i=2), a dedicateddepressed index cladding region (i=3) and a dedicated outer claddingregion (i=4). For purposes of illustration, the radial positions r_(1,1)and r_(1,2) refer to the outer radius r₁ (i=1) of a first core region(j=1) and the outer radius r₁ (i=1) of a second core region (j=2) of amulticore glass fiber, respectively. Similarly, the relative refractiveindices Δ_(1,1) and Δ_(1,2) refer to the relative refractive index Δ₁(i=1) of a first core region (j=1) and the relative refractive index Δ₁(i=1) of a second core region (j=2) of a multicore glass fiber,respectively. When a symbol designating a radial position or relativerefractive index includes a single subscript, it is understood that thesymbol and subscript refer to any of the regions of type i in themulticore glass fiber, where it is further understood that the numericalvalue associated with the symbol and subscript may be the same ordifferent for the different regions of type i in the multicore glassfiber. For example, the radial position r₁ refers to the outer radius ofany of the core regions of the multicore glass fiber, where it isunderstood that the numerical value of the outer radius r₁ may be thesame or different for any two of the different core regions in themulticore glass fiber.

The relative refractive index Δ₁(r) has a maximum value Δ_(1max) and aminimum value Δ_(1min). The relative refractive index Δ₂(r) has amaximum value Δ_(2max) and a minimum value Δ_(2min). The relativerefractive index Δ₃(r) has a maximum value Δ_(3max) and a minimum valueΔ_(3min). The relative refractive index Δ_(4doc)(r) has a maximum valueΔ_(4docmax) and a minimum value Δ_(4docmin). The relative refractiveindex Δ₄(R) has a maximum value Δ_(4max) and a minimum value Δ_(4min).The relative refractive index Δ₅(R) has a maximum value Δ_(5max) and aminimum value Δ_(5min). In embodiments in which the relative refractiveindex is constant or approximately constant over a region (e.g. astep-index profile), the maximum and minimum values of the relativerefractive index are equal or approximately equal. Unless otherwisespecified, if a single value is reported for the relative refractiveindex of a region (dedicated or common), the single value corresponds toan average value for the region. For core regions with an α-profile orgraded-index relative refractive index profile, Δ_(1max) corresponds tothe value of Δ₁ at the centerline (r=0) of the core region in someembodiments. In some embodiments, Δ_(1max) is offset from the centerline(r=0) of the core (e.g. a centerline dip in relative refractive indexmay be present).

It is understood that a core region is substantially cylindrical inshape and that a dedicated inner cladding region, a dedicated depressedindex cladding region, a dedicated outer cladding region, a commonexterior cladding region, a primary coating, and a secondary coating aresubstantially annular in shape. Common interior cladding regions haveshapes with internal cavities sized to accommodate the two or more coresand any dedicated cladding regions accompanying the two or more cores.The outer surface of a common interior cladding region preferably has acircular circumference that defines the radius R₄. Annular regions arecharacterized in terms of an inner radius and an outer radius. Radialpositions r₁, r₂, r₃, and r_(4doc), refer herein to the outermost radiiof a core region, a dedicated inner cladding region, a dedicateddepressed index cladding region, and a dedicated outer cladding,respectively. The glass fiber of the multicore optical fiber ispreferably substantially cylindrical in shape and R₅ refers to the outerradius of the glass fiber, which corresponds to the outer radius of thecommon exterior cladding. In some embodiments, the glass fiber issurrounded by a primary coating and a secondary coating, each of whichis substantially annular in shape. The radius R₆ refers to the outerradius of the primary coating and the radius R₇ refers to the outerradius of the secondary coating.

When two dedicated cladding regions are directly adjacent to each other,the outer radius of the inner of the two dedicated cladding regionscoincides with the inner radius of the outer of the two dedicatedcladding regions. In one embodiment, for example, the glass fiberincludes a core with a dedicated inner cladding region surrounded by anddirectly adjacent to a dedicated depressed index cladding region. Insuch an embodiment, the radius r₂ corresponds to the outer radius of thededicated inner cladding region and the inner radius of the dedicateddepressed index cladding region. In embodiments in which the relativerefractive index profile includes a dedicated inner cladding regiondirectly adjacent to the core, the radial position r₁ corresponds to theouter radius of the core and the inner radius of the dedicated innercladding region.

The following terminology applies to embodiments in which the relativerefractive index profile includes a dedicated inner cladding regionsurrounding and directly adjacent to a core region, a common interiorcladding region surrounding and directly adjacent to the dedicated innercladding region, and a common exterior cladding region surrounding anddirectly adjacent to the common interior cladding region. The differencer₂−r₁ between radial position r₂ and radial position r₁ is referred toherein as the thickness of the dedicated inner cladding region. Thedifference R₅−R₄ between radial position R₅ and radial position R₄ isreferred to herein as the thickness of the common exterior claddingregion.

The following terminology applies to embodiments in which the relativerefractive index profile includes a dedicated inner cladding regionsurrounding and directly adjacent to a core region, a dedicateddepressed index cladding region surrounding and directly adjacent to thededicated inner cladding region, a common interior cladding regionsurrounding and directly adjacent to the dedicated depressed indexcladding region, and a common exterior cladding region surrounding anddirectly adjacent to the common interior cladding region. The differencer₂−r₁ between radial position r₂ and radial position r₁ is referred toherein as the thickness of the dedicated inner cladding region. Thedifference r₃−r₂ between radial position r₃ and radial position r₂ isreferred to herein as the thickness of the dedicated depressed indexcladding region. The difference R₅−R₄ between radial position R₅ andradial position R₄ is referred to herein as the thickness of the commonexterior cladding region.

In embodiments with a primary coating surrounding and directly adjacentto a common exterior cladding region, and a secondary coatingsurrounding and directly adjacent the primary coating, the differenceR₆−R₅ between radial position R₆ and radial position R₅ is referred toherein as the thickness of the primary coating and the difference R₇−R₆between radial position R₇ and radial position R₆ is referred to hereinas the thickness of the secondary coating.

As will be described further hereinbelow, the relative refractiveindices of the core region, dedicated inner cladding region, dedicateddepressed index cladding region, dedicated outer cladding region, commoninterior cladding region and common exterior cladding region may differ.The relative refractive index of the core region is higher than therelative refractive index of any of the dedicated or common claddingregions. The relative refractive index of an inner cladding region maybe greater than, less than or equal to the relative refractive index ofa common interior cladding region. In embodiments that include adedicated inner cladding region, a dedicated depressed index claddingregion, a common interior cladding region and a common exterior claddingregion, the relative refractive index of the dedicated depressed indexcladding region is less than the relative refractive index of thededicated inner cladding region and the relative refractive index of thecommon interior cladding region. Any or all of the relative refractiveindices, radial positions, and thicknesses of the core region, dedicatedinner cladding region, dedicated depressed index cladding region, anddedicated outer cladding region for different cores of the multicoreoptical fiber may be the same or different. In different embodiments,all cores of the multicore optical fiber have step-index profiles, allcores of the multicore optical fiber have graded-index profiles, or somecores of the multicore optical fiber have step-index profiles and othercores of the multicore optical fiber have graded-index profiles.

Each of the regions may be formed from doped or undoped silica glass.Variations in refractive index relative to undoped silica glass areaccomplished by incorporating updopants or downdopants at levelsdesigned to provide a targeted refractive index or refractive indexprofile using techniques known to those of skill in the art. Updopantsare dopants that increase the refractive index of the glass relative tothe undoped glass composition. Downdopants are dopants that decrease therefractive index of the glass relative to the undoped glass composition.In one embodiment, the undoped glass is silica glass. When the undopedglass is silica glass, updopants include Cl, Br, Ge, Al, P, Ti, Zr, Nb,and Ta, and downdopants include F and B. Regions of constant refractiveindex may be formed by not doping or by doping at a uniformconcentration over the thickness of the region. Regions of variablerefractive index are formed through non-uniform spatial distributions ofdopants over the thickness of a region and/or through incorporation ofdifferent dopants in different regions.

The term “mode” refers to guided mode. A single-mode fiber is an opticalfiber designed to support only the fundamental LP01 modes over asubstantial length of the optical fiber (e.g., at least several meters),but that under certain circumstances can support multiple modes overshort distances (e.g., tens of centimeters). We assume that thebirefringence of the fiber is sufficiently low to assume that the twoorthogonally polarized components of the LP01 mode are degenerate andpropagate with the same phase velocity. A multimode optical fiber is anoptical fiber designed to support the fundamental LP01 mode and at leastone higher-order LP_(nm) mode over a substantial length of the opticalfiber, where either n≠0 or m≠1.

The present disclosure provides multicore glass fibers and multicoreoptical fibers as well as ribbons and cables containing multicore glassfibers and multicore optical fibers. In a ribbon, the multicore glassfibers or multicore optical fibers are aligned relative to one anotherin a substantially planar and parallel relationship. The multicore glassfibers or multicore optical fibers in ribbons are encapsulated by aribbon matrix in any of several known configurations (e.g., edge-bondedribbon, thin-encapsulated ribbon, thick-encapsulated ribbon, ormulti-layer ribbon) by conventional methods of making fiber opticribbons. The ribbon contains two or more multicore glass fibers ormulticore optical fibers. In some embodiments, the ribbon contains fouror more, or eight or more, or twelve or more, or sixteen or moremulticore glass fibers or multicore optical fibers. The ribbon matrixhas tensile properties similar to the tensile properties of a secondarycoating and is formed from the same, similar, or different compositionused to prepare a secondary coating. A cable includes a plurality ofmulticore glass fibers or multicore optical fibers surrounded by ajacket. The jacket typically has a circular cross-section and isflexible or rigid depending on the application requirement. Multicoreglass fibers or multicore optical fibers are densely or loosely packedinto a conduit enclosed by an inner surface of the jacket. The number offibers placed in the jacket is referred to as the “fiber count” ofcable. The jacket is formed from an extruded polymer material and mayinclude multiple concentric layers of polymers or other materials. Thecable may also include one or more strengthening members embedded withinthe jacket or placed within the conduit defined by the inner surface ofthe jacket. Strengthening members include fibers or rods that are morerigid than the jacket. The strengthening member is made from metal,braided steel, glass-reinforced plastic, fiber glass, or other suitablematerial. The cable may include other layers surrounded by the jacket(e.g. armor layers, moisture barrier layers, rip cords, etc.). The cablemay have a stranded, loose tube core or other fiber optic cableconstruction.

For purposes of illustration, the disclosure that follows describesmulticore glass fibers having two cores. It should be apparent, however,that multicore glass fibers having more than two cores are similarlycontemplated and within the scope of the disclosure. The number of coresin the multicore fiber is two or more, or three or more, or four ormore, or six or more, or eight or more, or twelve or more, or sixteen ormore, or between 2 and 32, or between 3 and 28, or between 4 and 24, orbetween 6 and 20, or between 8 and 16. Particular characteristics of thearrangement of cores that minimize crosstalk between cores or tunnelingoutside of the glass fiber are also described. These characteristicsapply similarly to any pair or combination of cores in multicore glassfibers having more than two cores. Although the disclosure emphasizesmulticore glass fibers, it is further understood that one or morepolymer coatings may be applied to the outer surface of the multicoreglass fiber. Polymer coatings include primary coatings, secondarycoatings, ink layers, and matrix materials known in the art.

One example of a relative refractive index profile is a step-indexrelative refractive index profile, which has a core region whoserefractive index is constant or approximately constant with distancefrom the centerline of the core. One example of a step-index fiber is afiber with a core region having a relative refractive index profile withan α-profile with a value of α greater than or equal to 10. Anotherexample of a relative refractive index profile is a graded-indexprofile, which has a core region whose refractive index varies withdistance from the centerline of the core. One example of a graded-indexfiber is a fiber with a core region having a relative refractive indexprofile with an α-profile with a value of α less than 10.

FIG. 1 illustrates a multicore glass fiber with two cores. In each core,a core region is surrounded by and directly adjacent to a dedicatedinner cladding region, which is surrounded by and directly adjacent to adedicated depressed index cladding region, which is surrounded by anddirectly adjacent to a common interior cladding region, which issurrounded by and directly adjacent to a common exterior claddingregion. A first core includes core region 20, dedicated inner claddingregion 28, dedicated depressed index cladding region 32, common interiorcladding region 36, and common exterior cladding region 37. Core region20 has an outer radius r₁ depicted at 24. Dedicated inner claddingregion 28 has an inner radius depicted at 24 and an outer radius r₂depicted at 28 a. Dedicated depressed index cladding region 32 has aninner radius depicted at 28 a and an outer radius n depicted at 32 a. Asecond core includes core region 30, dedicated inner cladding region 38,dedicated depressed index cladding region 42, common interior claddingregion 36, and common exterior cladding region 37. Core region 30 has anouter radius r₁ depicted at 34. Dedicated inner cladding region 38 hasan inner radius depicted at 34 and an outer radius r₂ depicted at 38 a.Dedicated depressed index cladding region 42 has an inner radiusdepicted at 38 a and an outer radius r₃ depicted at 42 a. Commoninterior cladding region 36 has a radius R₄ depicted at 14 and commonexterior cladding region 37 has a radius R₅ depicted at 15. The corespacing between the centerlines of core regions 20 and 30 is depicted at16. As used herein, “core spacing” of a core refers to the shortestdistance between the centerline of the core and the centerline ofanother core. Cores positioned at the core spacing are said to be“adjacent” to each other and are referred to herein as “adjacent cores”or a pair of adjacent cores. The core spacings of the cores in amulticore optical fiber are preferably the same, but may be different.The edge spacing between the centerline of core region 20 and the outersurface of common exterior cladding region 37 is depicted at 17. Theedge spacing between the centerline of core region 30 and the outersurface of common exterior cladding region 37 is depicted at 18. As usedherein, “edge spacing” of a core refers to the shortest distance betweenthe centerline of the core and the outer surface of the common exteriorcladding region (the surface defined by the radius R₅). Edge spacings 17and 18 may be the same or different. In embodiments with three or morecores, the edge spacing of any pair of cores may be the same ordifferent.

FIG. 2 depicts a relative refractive index profile for cores of amulticore glass fiber having a dedicated inner cladding region, adedicated depressed index cladding region, a common interior claddingregion and a common exterior cladding region as shown, for example, inFIG. 1. Relative refractive index profile 10 depicts core regions 20, 30with radius r₁ depicted at 24, 34; dedicated inner cladding regions 28,38 with radius r₂ depicted at 28 a, 38 a and thickness r₂−r₁ depicted at28 d, 38 d; dedicated depressed index cladding regions 32, 42 withradius r₃ depicted at 32 a, 42 a and thickness r₃−r₂ depicted at 28 w,38 w; common interior cladding region 36 with radius R₄ as depicted; andcommon exterior cladding region 37 with radius R₅ (not shown). Δ₁, Δ₂,Δ₃, Δ₄, and Δ₅ for the regions are also shown. Core regions 20, 30 eachhave a step-index relative refractive index profile. Although the radiir₁, r₂ and r₃ and relative refractive indices Δ₁, Δ₂, and Δ₃ aredepicted as being the same for core regions 20, 30 in FIG. 2, it isunderstood that in other embodiments any or all of r₁, r₂, r₃, Δ₁, Δ₂,and Δ₃ may differ for core regions 20, 30. The radii R₄ and R₅, and therelative refractive indices Δ₄ and Δ₅ are the same for all core regionsof the multicore optical fiber. In the embodiment of FIG. 2, therelative refractive indices Δ₃ and Δ₅ are depicted as being the same. Itis understood that in other embodiments, Δ₃>Δ₅ or Δ₃<Δ₅. In theembodiment of FIG. 2, the relative refractive indices Δ₂ and Δ₄ aredepicted as being the same. It is understood that in other embodiments,Δ₂>Δ₄ or Δ₂<Δ₄.

FIG. 3 depicts a relative refractive index profile for cores of amulticore glass fiber having a dedicated inner cladding region, adedicated depressed index cladding region, a common interior claddingregion, and a common exterior cladding region as shown, for example, inFIG. 1. Relative refractive index profile 10 depicts core regions 20, 30with radius r₁ depicted at 24, 34; dedicated inner cladding regions 28,38 with radius r₂ depicted at 28 a, 38 a and thickness r₂−r₁ depicted at28 d, 38 d; dedicated depressed index cladding regions 32, 42 withradius r₃ depicted at 32 a, 42 a and thickness r₃−r₂ depicted at 28 w,38 w; common interior cladding region 36 with radius R₄ as depicted; andcommon exterior cladding region 37 with radius R₅ (not shown). Δ₁, Δ₂,Δ₃, Δ₄, and Δ₅ for the regions are also shown. In one embodiment, coreregions 20, 30 each have a graded-index relative refractive indexprofile. Although the radii r₁, r₂ and r₃ and relative refractiveindices Δ₁, Δ₂, and Δ₃ are depicted as being the same for core regions20, 30 in FIG. 3, it is understood that in other embodiments any or allof r₁, r₂, r₃, Δ₁, Δ₂, and Δ₃ may differ for core regions 20, 30. Theradii R₄ and R₅, and the relative refractive indices Δ₄ and Δ₅ are thesame for all core regions of the multicore optical fiber. In theembodiment of FIG. 3, the relative refractive indices Δ₃ and Δ₅ aredepicted as being the same. It is understood that in other embodiments,Δ₃>Δ₅ or Δ₃<Δ₅. In the embodiment of FIG. 3, the relative refractiveindices Δ₂ and Δ₄ are depicted as being the same. It is understood thatin other embodiments, Δ₂>Δ₄ or Δ₂<Δ₄.

FIG. 4 illustrates a multicore glass fiber with two cores. In each core,a core region is surrounded by and directly adjacent to a dedicatedinner cladding region, which is surrounded by and directly adjacent to acommon interior cladding region, which is surrounded by and directlyadjacent to a common exterior cladding region. A first core includescore region 20, dedicated inner cladding region 28, common interiorcladding region 36, and common exterior cladding region 37. Core region20 has an outer radius r₁ depicted at 24. Dedicated inner claddingregion 28 has an inner radius depicted at 24 and an outer radius r₂depicted at 28 a. A second core includes core region 30, dedicated innercladding region 38, common interior cladding region 36, and commonexterior cladding region 37. Core region 30 has an outer radius r₁depicted at 34. Dedicated inner cladding region 38 has an inner radiusdepicted at 34 and an outer radius r₂ depicted at 38 a. Common interiorcladding region 36 has a radius R₄ depicted at 14 and common exteriorcladding region 37 has a radius R₅ depicted at 15. The core spacingbetween the centerlines of core regions 20 and 30 is depicted at 16. Asused herein, “core spacing” of a core refers to the shortest distancebetween the centerline of the core and the centerline of another core.The edge spacing between the centerline of core region 20 and the outersurface of common exterior cladding region 37 is depicted at 17. Theedge spacing between the centerline of core region 30 and the outersurface of common exterior cladding region 37 is depicted at 18. As usedherein, “edge spacing” of a core refers to the shortest distance betweenthe centerline of the core and the outer surface of the common exteriorcladding region (the surface defined by the radius R₅). Edge spacings 17and 18 may be the same or different. In embodiments with three or morecores, the edge spacing of any pair of cores may be the same ordifferent.

FIG. 5 depicts a relative refractive index profile for cores of amulticore glass fiber having a dedicated inner cladding region, a commoninterior cladding region and a common exterior cladding region as shown,for example, in FIG. 4. Relative refractive index profile 10 depictscore regions 20, 30 with radius r₁ depicted at 24, 34; dedicated innercladding regions 28, 38 with radius r₂ depicted at 28 a, 38 a andthickness r₂−r₁ depicted at 28 d, 38 d; common interior cladding region36 with radius R₄ as depicted; and common exterior cladding region 37with radius R₅ (not shown). Δ₁, Δ₂, Δ₄, and Δ₅ for the regions are alsoshown. Core regions 20, 30 each have a step-index relative refractiveindex profile. Although the radii r₁ and r₂ and relative refractiveindices Δ₁, and Δ₂ are depicted as being the same for core regions 20,30 in FIG. 2, it is understood that in other embodiments any or all ofr₁, r₂, Δ₁, and Δ₂ may differ for core regions 20, 30. The radii R₄ andR₅, and the relative refractive indices Δ₄ and Δ₅ are the same for allcore regions of the multicore optical fiber. In the embodiment of FIG.5, the relative refractive indices Δ₂ and Δ₅ are depicted as being thesame. It is understood that in other embodiments, Δ₂>Δ₅ or Δ₂<Δ₅.

FIG. 6 depicts a relative refractive index profile for cores of amulticore glass fiber having a dedicated inner cladding region, a commoninterior cladding region, and a common exterior cladding region asshown, for example, in FIG. 4. Relative refractive index profile 10depicts core regions 20, 30 with radius r₁ depicted at 24, 34; dedicatedinner cladding regions 28, 38 with radius r₂ depicted at 28 a, 38 a andthickness r₂−r₁ depicted at 28 d, 38 d; common interior cladding region36 with radius R₄ as depicted; and common exterior cladding region 37with radius R₅ (not shown). Δ₁, Δ₂, Δ₄, and Δ₅ for the regions are alsoshown. In one embodiment, core regions 20, 30 each have a graded-indexrelative refractive index profile. Although the radii r₁ and r₂ andrelative refractive indices Δ₁ and Δ₂ are depicted as being the same forcore regions 20, 30 in FIG. 6, it is understood that in otherembodiments any or all of r₁, r₂, Δ₁, and Δ₂ may differ for core regions20, 30. The radii R₄ and R₅, and the relative refractive indices Δ₄ andΔ₅ are the same for all core regions of the multicore optical fiber. Inthe embodiment of FIG. 6, the relative refractive indices Δ₂ and Δ₅ aredepicted as being the same. It is understood that in other embodiments,Δ₂>Δ₅ or Δ₂<Δ₅.

FIG. 7 illustrates a multicore glass fiber with two cores. In each core,a core region is surrounded by and directly adjacent to a commoninterior cladding region, which is surrounded by and directly adjacentto a common exterior cladding region. A first core includes core region20, common interior cladding region 36, and common exterior claddingregion 37. Core region 20 has an outer radius r₁ depicted at 24. Asecond core includes core region 30, common interior cladding region 36,and common exterior cladding region 37. Core region 30 has an outerradius r₁ depicted at 34. Common interior cladding region 36 has aradius R₄ depicted at 14 and common exterior cladding region 37 has aradius R₅ depicted at 15. The core spacing between the centerlines ofcore regions 20 and 30 is depicted at 16. As used herein, “core spacing”of a core refers to the shortest distance between the centerline of thecore and the centerline of another core. The edge spacing between thecenterline of core region 20 and the outer surface of common exteriorcladding region 37 is depicted at 17. The edge spacing between thecenterline of core region 30 and the outer surface of common exteriorcladding region 37 is depicted at 18. As used herein, “edge spacing” ofa core refers to the shortest distance between the centerline of thecore and the outer surface of the common exterior cladding region (thesurface defined by the radius R₅). Edge spacings 17 and 18 may be thesame or different. In embodiments with three or more cores, the edgespacing of any pair of cores may be the same or different.

FIG. 8 depicts a relative refractive index profile for cores of amulticore glass fiber having a common interior cladding region and acommon exterior cladding region as shown, for example, in FIG. 7.Relative refractive index profile 10 depicts core regions 20, 30 withradius r₁ depicted at 24, 34; common interior cladding region 36 withradius R₄ as depicted; and common exterior cladding region 37 withradius R₅ (not shown). Δ₁, Δ₄, and Δ₅ for the regions are also shown.Core regions 20, 30 each have a step-index relative refractive indexprofile. Although the radius r₁ and relative refractive index Δ₁ aredepicted as being the same for core regions 20, 30 in FIG. 8, it isunderstood that in other embodiments any or all of r₁ and Δ₁ may differfor core regions 20, 30. The radii R₄ and R₅, and the relativerefractive indices Δ₄ and Δ₅ are the same for all core regions of themulticore optical fiber.

FIG. 9 depicts a relative refractive index profile for cores of amulticore glass fiber having a common interior cladding region, and acommon exterior cladding region as shown, for example, in FIG. 7.Relative refractive index profile 10 depicts core regions 20, 30 withradius r₁ depicted at 24, 34; common interior cladding region 36 withradius R₄ as depicted; and common exterior cladding region 37 withradius R₅ (not shown). Δ₁, Δ₄, and Δ₅ for the regions are also shown. Inone embodiment, core regions 20, 30 each have a graded-index relativerefractive index profile. Although the radius r₁ and relative refractiveindex Δ₁ are depicted as being the same for core regions 20, 30 in FIG.9, it is understood that in other embodiments any or all of r₁ and Δ₁may differ for core regions 20, 30. The radii R₄ and R₅, and therelative refractive indices Δ₄ and Δ₅ are the same for all core regionsof the multicore optical fiber.

In some embodiments, the multicore glass fiber includes a core with astep-index relative refractive index profile and a core with agraded-index (e.g. α-profile) relative refractive index profile.

As noted above, radii (r₁, r₂, and/or r₃), relative refractive indicesof the core regions and dedicated cladding regions (Δ₁, Δ₂, and/or Δ₃),and core profile (step index vs. graded index) may be the same ordifferent for any combination of the two or more cores. Differences in αand Δ₁, Δ₂, and Δ₃ are achievable through differences in composition,dopant concentration, spatial distribution of dopant, and/or dopanttype. Differences in r₁, r₂, and r₃ are achievable by controlling thethicknesses of regions in a preform corresponding to the core region,inner cladding region, and/or depressed index cladding region.

The relative refractive index profiles of the core regions and claddingregions, and the spacing between centerlines of the cores of themulticore optical fiber are selected to minimize crosstalk betweencores. As used herein, crosstalk refers to transfer of optical signalintensity from one core to another core. Crosstalk is facilitated byoverlap of the evanescent field of an optical signal in one core with acladding region or core region of another core. Crosstalk is alsofacilitated by overlap of the evanescent field of an optical signal inone core with the evanescent field of an optical signal in another corein a cladding region between the two cores.

Crosstalk is schematically depicted in FIG. 10, which shows multicoreglass fiber 60 having cores 62 and 64. Cores 62 and 64 includeembodiments having any of the core regions and dedicated claddingregions described herein (details not shown in FIG. 10). Cores 62 and 64are surrounded by common interior cladding 66 and common exteriorcladding 67. Pulsed laser 68 launches primary signal 70 into core 62 atinput end 71 of multicore glass fiber 60. As primary signal 70propagates along core 62, crosstalk can occur and a portion of primarysignal 70 can be transferred to core 64. The transfer of optical signalis depicted as transfer signal 72 and leads to production of crosstalksignal 74 in core 64. Crosstalk signal 74 propagates through core 64 andis detected by detector 76 at output end 73 of multicore glass fiber 60.Crosstalk leads to two undesirable effects in multicore fibers. First,crosstalk leads to a reduction of the power of primary signal 70detected for core 62 at output end 73 of multicore glass fiber 60.Second, if a secondary signal (not shown in FIG. 10) is launched intocore 64 at input end 71 of multicore glass fiber 60, crosstalk signal 74from core 62 interferes or mixes with the secondary signal as itpropagates through core 64, causing distortions or errors in thedetection of the secondary signal as it exits core 64 at the output end73 of multicore glass fiber 60. It is recognized that back crosstalkfrom core 64 to core 62 may occur so that transfer signal 72 correspondsto the net transfer of optical signal from core 62 to core 64 in thecrosstalk process. For weak coupling, the back crosstalk is negligible.

To maintain purity of the optical signals launched into the differentcores of a multicore glass fiber, it is preferably to configure themulticore optical fiber to provide a low degree of crosstalk (weakcoupling) between cores. The degree of crosstalk is a measure of thepower of the crosstalk signal relative to the power of the signal fromwhich it originates (e.g. the power of crosstalk signal 74 relative toprimary signal 70). As used herein, degree of crosstalk is normalized tothe distance traversed by the crosstalk signal and the primary signal inthe direction of propagation of the optical signal (see Eq. (7) below).Degree of crosstalk is expressed herein in units of dB/km, or dB/10 km,or dB/100 km.

By way of example, the direction of propagation of primary signal 70 andcrosstalk signal 74 shown in FIG. 10 is the z-direction. The z-directionis parallel to the centerlines of cores 62 and 64 and is oriented in thedirection from the input (launch end) to the output (detection end) ofthe multicore glass fiber. At each z location over a distance of Δz,crosstalk occurs through transfer of a portion of primary signal 70 fromcore 62 to core 64 to provide crosstalk signal 74. The crosstalk isdepicted as transfer signal 72. If the power of crosstalk signal 74 isP₂ at location z>z₁ and the power of primary signal 70 at location z₁ isP₁, the degree of crosstalk at location z in a linear scale is given as:

$\begin{matrix}{{{Degree}\mspace{14mu}{of}\mspace{14mu}{Crosstalk}\mspace{14mu}(z)} = \frac{P_{2}\text{/}P_{1}}{z - z_{1}}} & (7)\end{matrix}$and the degree of crosstalk in a logarithmic scale is given as:

$\begin{matrix}{{{Degree}\mspace{14mu}{of}\mspace{14mu}{Crosstalk}\mspace{14mu}(z)\left( {{in}\mspace{14mu}{dB}} \right)} = {10{\log\left( \frac{P_{2}\text{/}P_{1}}{z - z_{1}} \right)}}} & (8)\end{matrix}$Note that the power of crosstalk signal 74 increases linearly with thedistance z only in the linear scale (Eq. (7)) and not in the logarithmicscale (Eq. (8)). For convenience, we use the logarithmic scale. Whenusing Eq. (8), we express z and z₁ in km (kilometers) and the degree ofcrosstalk as dB/km. For a fiber of arbitrary length L (expressed inunits of km), the degree of crosstalk in dB normalized to the length Lis calculated by

$\begin{matrix}{{{Degree}\mspace{14mu}{of}\mspace{14mu}{{Crosstalk}\mspace{14mu}\left\lbrack {{dB}\text{/}L} \right\rbrack}} = {{{Degree}\mspace{14mu}{of}\mspace{14mu}{Crosstalk}\mspace{14mu}\left( \frac{dB}{km} \right)} + {10\log L}}} & (9)\end{matrix}$In Eq. (9), the term “Degree of Crosstalk (dB/km)” refers to the degreeof crosstalk computed from Eq. (8) for a fiber length of 1 km.Application of Eq. (9) is now described. If, for example, the power P₂of crosstalk signal 74 is 0.001% of the power P₁ of primary signal 70for a 1 km separation (z-z₁) between crosstalk signal 74 and primarysignal 70, the degree of crosstalk is −50 dB/km (Eq. (8)). If we applythis degree of crosstalk to a 1 m long fiber segment, the degree ofcrosstalk computed from Eq. (9) is −80 dB/10⁻³ km, which is equivalentto −80 dB/m. If we apply this degree of crosstalk to a 100 km long fibersegment, the degree of crosstalk computed from Eq. (9) is −30 dB/100 km.

For the present multicore optical fibers, the degree of crosstalk at1310 nm between any two cores of the multicore optical fiber is lessthan −40 dB/km, or less than −45 dB/km, or less than −50 dB/km, or lessthan −55 dB/km, or less than −60 dB/km, or in the range from −90 dB/kmto −40 dB/km, or in the range from −80 dB/km to −45 dB/km, or in therange from −75 dB/km to −50 dB/km.

For the present multicore optical fibers, the degree of crosstalk at1550 nm between any two cores of the multicore optical fiber is lessthan −25 dB/km, or less than −35 dB/km, or less than −45 dB/km, or lessthan −50 dB/km, or less than −55 dB/km, or in the range from −75 dB/kmto −25 dB/km, or in the range from −65 dB/km to −30 dB/km, or in therange from −55 dB/km to −35 dB/km.

Configurations of cores in the multicore glass fiber are now described.Relevant considerations in the configuration of cores include relativerefractive indices (Δ₁, Δ₂, Δ₃, Δ₄, Δ₅, and minimum and maximum valuesof each), radial positions of core and cladding regions (r₁, r₂, r₃, R₄and R₅), core spacing, and edge spacing.

The relative ordering of relative refractive indices in the relativerefractive index profile shown in FIGS. 2 and 3 satisfy the conditionsΔ₁ (or Δ_(1max))>Δ₄>Δ₃ (or Δ_(3min)), Δ₁ (or Δ_(1max))>Δ₂>Δ₃ (orΔ_(3min)), Δ₁ (or Δ_(1max))>Δ₄>Δ₅ (or Δ_(5min)), Δ₁ (or Δ_(1max))>Δ₂>Δ₅(or Δ_(5min)). The values of Δ₂ and Δ₄ may be equal or either may begreater than the other, but both Δ₂ and Δ₄ are between Δ₁ (or Δ_(1max))and Δ₃ (or Δ_(3min)) and are also between Δ₁ (or Δ_(1max)) and Δ₅ (orΔ_(5min)). The values of Δ₃ and Δ₅ may be equal or either may be greaterthan the other, but both Δ₃ and Δ₅ are less than Δ₂ and Δ₄.

The relative ordering of relative refractive indices Δ₁, Δ₃, and Δ₄ inthe relative refractive index profiles shown in FIGS. 5 and 6 satisfythe conditions Δ₁ (or Δ_(1max))>Δ₄>Δ₂ (or Δ_(2min)) and Δ₁ (orΔ_(1max))>Δ₄>Δ₅ (or Δ_(5min)). The values of Δ₂ and Δ₅ may be equal oreither may be greater than the other, but both Δ₂ and Δ₅ are less thanΔ₄.

The relative ordering of relative refractive indices Δ₁, Δ₄, and Δ₅ inthe relative refractive index profiles shown in FIGS. 8 and 9 satisfythe conditions Δ₁ (or Δ_(1max))>Δ₄>Δ₅ (or Δ_(5min)).

Each of the plurality of core regions of the multicore glass fibercomprises silica glass. The silica glass is undoped silica glass,updoped silica glass, and/or downdoped silica glass. Updoped silicaglass includes silica glass doped with one or more of GeO₂, an alkalimetal oxide (e.g. Na₂O, K₂O, Li₂O, Cs₂O, or Rb₂O) or a halogen (e.g. Cl,Br). In embodiments, the concentration of GeO₂ in silica glass is from 5wt % to 22 wt %, or 7 wt % to 15 wt %. Downdoped silica glass includessilica glass doped with one or more of F or B. The concentration of K₂Oin certain embodiments of the core regions, expressed in terms of theamount of K, is in the range from 20 ppm-1000 ppm, or 35 ppm-500 ppm, or50 ppm-300 ppm, where ppm refers to parts per million by weight. Alkalimetal oxides other than K₂O are present in amounts corresponding to theequivalent molar amount of K₂O as determined from the amount of Kindicated above in other embodiments. The concentration of Cl or Br insome embodiments of the core regions is in the range from 0.5 wt %-6.0wt %, or in the range from 1.0 wt %-5.5 wt %, or in the range from 1.5wt %-5.0 wt %, or in the range from 2.0 wt %-4.5 wt %, or in the rangefrom 2.5 wt %-4.0 wt %. The dopant type and/or dopant concentration indifferent core regions of the multicore glass fiber are the same ordifferent.

In some embodiments, at least one core region of the multicore glassfiber includes an updopant and a downdopant, where the concentration ofupdopant is highest at the centerline (r=0) and lowest at the coreradius r₁ and the concentration of downdopant is lowest at thecenterline (r=0) and highest at the core radius r₁. In such embodiments,the relative refractive index Δ₁ can have a positive value near thecenterline (r=0) and decrease to a negative value at the core radius r₁.

In some embodiments, the relative refractive index of at least one coreregion of the plurality of core regions of the multicore glass fiber isdescribed by an α-profile with an α value in the range from 1.5-10, orin the range from 1.7-8.0, or in the range from 1.8-6.0, or in the rangefrom 1.9-5.0, or in the range from 2.0-4.0, or in the range from 10-50,or in the range from 11-40, or in the range from 12-30. As the value ofα increases, the relative refractive profile more closely approaches astep-index profile. For purposes of the present disclosure, relativerefractive index profiles with values of α≥10 are regarded as step-indexprofiles and relative refractive index profiles with values of α<10 areregarded as graded-index profiles.

For purposes of the present disclosure, the outer radius r₁ of each coreof the multicore optical fiber is defined as the radial coordinate atwhich the relative refractive index Δ₁ has decreased from a maximumvalue Δ_(1max) to Δ₂. The outer radius r₁ of each of the core regions ofthe multicore glass fiber is in the range from 3.0 μm-9.0 μm, or in therange from 3.0 μm-8.0 μm, or in the range from 3.0 μm-7.0 μm, or in therange from 3.5 μm-6.5 μm.

The relative refractive index Δ₁ or Δ_(1max) of each of the core regionsof the multicore glass fiber is in the range from 0.10%-2.0%, or in therange from 0.20%-1.5%, or in the range from 0.30%-1.0%, or in the rangefrom 0.40%-0.80%, or in the range from 0.20%-0.80%, or in the range from0.30%-0.60%.

A difference Δ_(1,1)−Δ_(1,2) between core region 1 and core region 2 ofa multicore glass fiber is less than 0.25%, or less than 0.20%, or lessthan 0.15%, or less than 0.10%, or less than 0.05%, or 0.00%, or in therange from 0.01%-0.25%, or in the range from 0.05%-0.25%, or in therange from 0.05%-0.20%. A magnitude of a difference Δ_(1,1)−Δ_(1,2)between core region 1 and core region 2 of a multicore glass fiber isless than 0.25%, or less than 0.20%, or less than 0.15%, or less than0.10%, or less than 0.05%, or in the range from 0.01%-0.25%, or in therange from 0.05%-0.25%, or in the range from 0.05%-0.20%.

A difference Δ_(1max,1)−Δ_(1max,2) between core region 1 and core region2 of a multicore glass fiber is less than 0.25%, or less than 0.20%, orless than 0.15%, or less than 0.10%, or less than 0.05%, or in the rangefrom 0.01%-0.25%, or in the range from 0.05%-0.25%, or in the range from0.05%-0.20%. A magnitude of a difference Δ_(1max,1)−Δ_(1max,2) betweencore region 1 and core region 2 of a multicore glass fiber is less than0.25%, or less than 0.20%, or less than 0.15%, or less than 0.10%, orless than 0.05%, or in the range from 0.01%-0.25%, or in the range from0.05%-0.25%, or in the range from 0.05%-0.20%.

In some embodiments, the relative refractive index of at least one ofthe plurality of core regions of the multicore glass fiber is describedby a step-index profile having a constant or approximately constantvalue corresponding to Δ_(1max).

In embodiments in which a core region of a multicore glass fiber isdirectly adjacent to a dedicated inner cladding region, the dedicatedinner cladding region is comprised of undoped silica glass, updopedsilica glass, or downdoped silica glass. Updoped silica glass includessilica glass doped with GeO₂, an alkali metal oxide (e.g. Na₂O, K₂O,Li₂O, Cs₂O, or Rb₂O) or a halogen (e.g. Cl, Br). Downdoped silica glassincludes silica glass doped with F or B. If doped with an updopant, theaverage concentration of updopant in the dedicated inner cladding regionis less than the average concentration of updopant in the core region.If doped with a downdopant, the average concentration of downdopant inthe dedicated inner cladding region is greater than the averageconcentration of downdopant in the core region.

In embodiments in which at least one core region of the multicore glassfiber is directly adjacent to a dedicated inner cladding region that isdirectly adjacent to a common interior cladding region, the relativerefractive index Δ₂ or Δ_(2max) or Δ_(2min) of the dedicated innercladding region is less than −0.30%, or less than −0.40%, or less than−0.50%, or less than −0.60%, or less than −0.70%, or less than −0.80%,or in the range from −1.1% to −0.10%, or in the range from −0.80% to−0.10%, or in the range from −0.80% to −0.30%, or in the range from−0.80% to −0.40%, or in the range from −0.70% to −0.20%, or in the rangefrom −0.65% to −0.30%, or in the range from −0.60% to −0.30%, or in therange from −0.20% to 0.20%, or in the range from −0.10% to 0.10%, or inthe range from −0.05% to 0.05%. The relative refractive index Δ₂ ispreferably constant or approximately constant. A difference Δ_(1max)−Δ₂(or a difference Δ_(1max)−Δ_(2max), or a difference Δ_(1max)−Δ_(2min))is greater than 0.10%, or greater than 0.20%, or greater than 0.30%, orgreater than 0.50%, or greater than 0.70%, or greater than 1.0%, or inthe range from 0.10%-1.5%, or in the range from 0.20%-1.2%, or in therange from 0.30%-1.0%.

In embodiments in which at least one core region of the multicore glassfiber is directly adjacent to a dedicated inner cladding region, theradius r₂ of a dedicated inner cladding region is in the range from 4.0μm-18.0 μm, or in the range from 4.5 μm-16.0 μm, or in the range from5.0 μm-14.0 μm, or in the range from 4.0 μm-10.0 μm, or in the rangefrom 5.0 μm-9.0 μm, or in the range from 9.0 μm-18.0 μm, or in the rangefrom 10.0 μm-17.0 μm, or in the range from 11.0 μm-16.0 μm. Thethickness r₂−r₁ of the inner cladding region is in the range from 1.0μm-14.0 μm, or in the range from 2.0 μm-12.0 μm, or in the range from3.0 μm-10.0 μm, or in the range from 1.0 μm-5.0 μm, or in the range from1.5 μm-4.5 μm, or in the range from 2.0 μm-4.0 μm, or in the range from8.0 μm-14.0 μm, or in the range from 8.5 μm-13.5 μm, or in the rangefrom 9.0 μm-13.0 μm.

In embodiments in which at least one core region of the multicore glassfiber is directly adjacent to a dedicated inner cladding region, thevalue of Δ₂ (or Δ_(2max)) for dedicated inner cladding regions ofdifferent core regions is the same or different.

In embodiments in which at least one core region of the multicore glassfiber is directly adjacent to a dedicated inner cladding region, thevalue of r₂ for dedicated inner cladding regions of different coreregions is the same or different.

In embodiments in which at least one core region of the multicore glassfiber is directly adjacent to a dedicated inner cladding region that isadjacent to a dedicated depressed index cladding region that is directlyadjacent to a common interior cladding region, the dedicated depressedindex cladding region comprises downdoped silica glass. The preferreddowndopant is F. The concentration of F is in the range from 0.1 wt%-2.5 wt %, or in the range from 0.25 wt %-2.25 wt %, or in the rangefrom 0.3 wt %-2.0 wt %.

In embodiments in which at least one core region of the multicore glassfiber is directly adjacent to a dedicated inner cladding region that isdirectly adjacent to a common interior cladding region, the dedicatedinner cladding region comprises downdoped silica glass. The preferreddowndopant is F. The concentration of F is in the range from 0.1 wt%-2.5 wt %, or in the range from 0.25 wt %-2.25 wt %, or in the rangefrom 0.3 wt %-2.0 wt %.

In embodiments in which at least one core region is directly adjacent toa dedicated inner cladding region that is directly adjacent to adedicated depressed index cladding region, the relative refractive indexΔ₃ or Δ_(3min) is less than −0.30%, or less than −0.40%, or less than−0.50%, or less than −0.60%, or less than −0.70%, or less than −0.80%,or in the range from −1.1% to −0.10%, or in the range from −0.80% to−0.10%, or in the range from −0.80% to −0.30%, or in the range from−0.80% to −0.40%, or in the range from −0.20% to −0.70%, or in the rangefrom −0.30% to −0.60%. The relative refractive index Δ₃ is preferablyconstant or approximately constant. A difference Δ_(1max)−Δ₃ (or adifference Δ_(1max)−Δ_(3min), or a difference Δ₁−Δ₃, or a differenceΔ₁−Δ_(3min)) is greater than 0.30%, or greater than 0.50%, or greaterthan 0.80%, or greater than 1.0%, or in the range from 0.30%-2.0%, or inthe range from 0.40%-1.7%, or in the range from 0.50%-1.4%. A differenceΔ₂−Δ₃ (or a difference Δ₂−Δ_(3min), or a difference Δ_(2max)−Δ₃, or adifference Δ_(2max)−Δ_(3min)) is greater than 0.20%, or greater than0.35%, or greater than 0.50%, or in the range from 0.20%-0.90%, or inthe range from 0.30%-0.80%.

In embodiments in which at least one core region is directly adjacent toa dedicated inner cladding region that is directly adjacent to adedicated depressed index cladding region, the inner radius of thededicated depressed index cladding region is r₂ and has the valuesspecified above. The outer radius r₃ of the dedicated depressed indexcladding region is in the range from 7.0 μm-20.0 μm, or in the rangefrom 8.5 μm-18.0 μm, or in the range from 10.0 μm-16.0 μm. The thicknessr₃−r₂ of the dedicated depressed index cladding region is in the rangefrom 1.0 μm-10.0 μm, or in the range from 1.5 μm-9.0 μm, or in the rangefrom or in the range from 2.0 μm-8.0 μm, or in the range from 3.0 μm-7.0μm.

In embodiments in which at least one core region of the multicore glassfiber is directly adjacent to a dedicated inner cladding region that isdirectly adjacent to a dedicated depressed index cladding region, thevalue of r₃ for dedicated depressed index cladding regions of differentcore regions is the same or different.

In embodiments in which at least one core region of the multicore glassfiber is directly adjacent to a dedicated inner cladding region that isdirectly adjacent to a dedicated depressed index cladding region, thevalue of Δ₃ (or Δ_(3min)) for dedicated depressed index cladding regionsof different core regions is the same or different.

In embodiments in which at least one core region is directly adjacent toa dedicated depressed index cladding region, the relative refractiveindex Δ₃ or Δ_(3min) is in the range from −0.10% to −0.80%, or in therange from −0.20% to −0.70%, or in the range from −0.30% to −0.60%.

In embodiments in which at least one core region is directly adjacent toa dedicated depressed index cladding region, the inner radius of thededicated depressed index cladding region is r₁ and has the valuesspecified above. The outer radius r₃ of the dedicated depressed indexcladding region is in the range from 7.0 μm-20.0 μm, or in the rangefrom 8.5 μm-18.0 μm, or in the range from 10.0 μm-16.0 μm. A thicknessr₃−r₁ of the dedicated depressed index cladding region is in the rangefrom 1.0 μm-10.0 μm, or in the range from 1.5 μm-9.0 μm, or in the rangefrom or in the range from 2.0 μm-8.0 μm, or in the range from 3.0 μm-7.0μm.

In embodiments in which at least one core region of the multicore glassfiber is directly adjacent to a dedicated depressed index claddingregion, the value of r₃ for dedicated depressed index cladding regionsof different core regions is the same or different.

In embodiments in which at least one core region of the multicore glassfiber is directly adjacent to a dedicated depressed index claddingregion, the value of Δ₃ (or Δ_(3min)) for dedicated depressed indexcladding regions of different core regions is the same or different.

The relative refractive index Δ₄ or Δ_(4max) of the common interiorcladding region is in the range from −0.30% to 0.30%, or in the rangefrom −0.20% to 0.20%, or in the range from −0.10% to 0.10%, or in therange from −0.05% to 0.05%. The relative refractive index Δ₄ ispreferably constant or approximately constant.

In embodiments in which a common interior cladding region surrounds andis adjacent to a dedicated depressed index cladding region, whichsurrounds and is directly adjacent to a dedicated inner cladding region,which surrounds and is directly adjacent to a core region, a differenceΔ₄−Δ₃ (or a difference Δ₄−Δ_(3min), or a difference Δ_(4max)−Δ₃, or adifference Δ_(4max)−Δ_(3min)) is greater than 0.10%, or greater than0.20%, or greater than 0.30%, or greater than 0.50%, or in the rangefrom 0.10%-0.80%, or in the range from 0.20%-0.70%.

In embodiments in which a common interior cladding region surrounds andis adjacent to a dedicated depressed index cladding region, whichsurrounds and is directly adjacent to a dedicated inner cladding region,which surrounds and is directly adjacent to a core region, a differenceΔ₄−Δ₅ (or a difference Δ₄−Δ_(5min), or a difference Δ_(4max)−Δ₅, or adifference Δ_(4max)−Δ_(5min)) is greater than 0.10%, or greater than0.20%, or greater than 0.30%, or greater than 0.50%, or in the rangefrom 0.10%-0.80%, or in the range from 0.20%-0.70%

In embodiments in which a common interior cladding region surrounds andis adjacent to a dedicated inner cladding region, which surrounds and isdirectly adjacent to a core region, a difference Δ₄−Δ₂ (or a differenceΔ₄−Δ_(2min), or a difference Δ_(4max)−Δ₂, or a differenceΔ_(4max)−Δ_(2min)) is greater than 0.10%, or greater than 0.20%, orgreater than 0.30%, or greater than 0.50%, or in the range from0.10%-0.80%, or in the range from 0.20%-0.70%.

In embodiments in which a common interior cladding region surrounds andis adjacent to a dedicated inner cladding region, which surrounds and isdirectly adjacent to a core region, a difference Δ₄−Δ₅ (or a differenceΔ₄−Δ_(5min), or a difference Δ_(4max)−Δ₅, or a differenceΔ_(4max)−Δ_(5min)) is greater than 0.10%, or greater than 0.20%, orgreater than 0.30%, or greater than 0.50%, or in the range from0.10%-0.80%, or in the range from 0.20%-0.70%.

The outer radius R₄ of a common interior cladding region is less than125.0 μm, or less than 100.0 μm, or less than 80.0 μm, or less than 65.0μm, or less than 62.5 μm, or less than 60.0 lam, or less than 57.5 μm orless than 55.0 μm, or less than 52.5 μm or in the range from 50.0μm-125.0 μm, or in the range from 55.0 μm-100.0 μm, or in the range from57.5 μm-80.0 μm, or in the range from 60.0 μm-70.0 μm, or in the rangefrom 50.0 μm-60.0 μm, or in the range from 52.5 μm-60.0 μm, or in therange from 55.0 μm-60.0 μm. The thickness R₄−r₂ of the common interiorcladding region (in embodiments in which the common interior claddingregion is directly adjacent to an inner cladding region that is directlyadjacent to a core region), or the thickness R₄−r₃ of the commoninterior cladding region (in embodiments in which the common interiorcladding region is directly adjacent to a dedicated depressed indexcladding region that is directly adjacent to a dedicated inner claddingregion that is directly adjacent to a core region) of the commoninterior cladding region is in the range from 20.0 μm-80.0 μm, or in therange from 25.0 μm-70.0 μm, or in the range from 30.0 μm-60.0 μm.

In embodiments in which at least one core region of the multicore glassfiber is directly adjacent to a dedicated inner cladding region that isdirectly adjacent to a common interior cladding region, the relativerefractive index Δ₅ or Δ_(5min) is less than −0.30%, or less than−0.40%, or less than −0.50%, or less than −0.60%, or less than −0.70%,or less than −0.80%, or in the range from −1.1% to −0.10%, or in therange from −0.80% to −0.10%, or in the range from −0.80% to −0.30%, orin the range from −0.80% to −0.40%, or in the range from −0.20% to−0.70%, or in the range from −0.30% to −0.60%. The relative refractiveindex Δ₅ is preferably constant or approximately constant. A differenceΔ_(1max)−Δ₅ (or a difference Δ_(1max)−Δ_(5min), or a difference Δ₁−Δ₅,or a difference Δ₁−Δ_(5min)) is greater than 0.30%, or greater than0.50%, or greater than 0.80%, or greater than 1.0%, or in the range from0.30%-2.0%, or in the range from 0.40%-1.7%, or in the range from0.50%-1.4%.

In embodiments in which at least one core region is directly adjacent toa dedicated inner cladding region that is directly adjacent to adedicated depressed index cladding region that is directly adjacent to acommon interior cladding region, the relative refractive index Δ₅ orΔ_(5min) is less than −0.30%, or less than −0.40%, or less than −0.50%,or less than −0.60%, or less than −0.70%, or less than −0.80%, or in therange from −1.1% to −0.10%, or in the range from −0.80% to −0.10%, or inthe range from −0.80% to −0.30%, or in the range from −0.80% to −0.40%,or in the range from −0.20% to −0.70%, or in the range from −0.30% to−0.60%. The relative refractive index Δ₅ is preferably constant orapproximately constant. A difference Δ_(1max)−Δ₅ (or a differenceΔ_(1max)−Δ_(5min), or a difference Δ₁−Δ₅, or a difference Δ₁−Δ_(5min))is greater than 0.30%, or greater than 0.50%, or greater than 0.80%, orgreater than 1.0%, or in the range from 0.30%-2.0%, or in the range from0.40%-1.7%, or in the range from 0.50%-1.4%. A difference Δ₂−Δ₅ (or adifference Δ₂−Δ_(5min), or a difference Δ_(2max)−Δ₅, or a differenceΔ_(2max)−Δ_(5min)) is greater than 0.20%, or greater than 0.35%, orgreater than 0.50%, or in the range from 0.20%-0.90%, or in the rangefrom 0.30%-0.80%.

The outer radius R₅ of a common exterior cladding region is less than130.0 μm, or less than 100.0 μm, or less than 80.0 μm, or less than 65.0μm, or less than 62.5 μm, or less than 60.0 μm, or less than 55.0 μm, orin the range from 50.0 μm-130.0 μm, or in the range from 55.0-100.0 μm,or in the range from 57.5 μm-80.0 μm, or in the range from 60.0 μm-70.0μm, or in the range from 55.0 μm-65.0 μm, or in the range from 58.0μm-63.0 μm, or in the range from 61.0 μm-63.0 μm. The thickness R₅−R₄ ofthe common exterior cladding region is in the range from 1.0 μm-30.0 μm,or in the range from 2.0 μm-20.0 μm, or in the range from 3.0 μm-15.0μm, or in the range from 4.0 μm-12.0 μm, or in the range from 5.0μm-10.0 μm.

A mode field diameter MFD of at least one core region (in combinationswith its cladding regions) of the multicore glass fiber is greater than7.0 μm, or greater than 7.5 μm, or greater than 8.0 μm, or greater than8.5 μm, or in the range from 7.0 μm-11.0 μm, or in the range from 7.5μm-10.0 μm, or in the range from 8.0 μm-9.5 μm at a wavelength of 1310nm.

A mode field diameter MFD of at least one core region (in combinationswith its cladding regions) of the multicore glass fiber is greater than7.5 μm, or greater than 8.0 μm, or greater than 8.5 nm, or greater than9.0 μm, or greater than 9.5 μm, or in the range from 7.5 μm-12.0 μm, orin the range from 7.5 μm-11.0 μm, or in the range from 8.0 μm-10.0 μm ata wavelength of 1550 nm.

An effective area Δ_(eff) of at least one core region (in combinationswith its cladding regions) of the multicore glass fiber is greater than35 μm², or greater than 40 μm², or greater than 45 μm², or greater than50 μm², or greater than 55 μm², or in the range from 35 μm²-90 μm², orin the range from 40 μm²-80 μm², or in the range from 45 μm²-75 μm², orin the range from 50 μm²-70 μm² at a wavelength of 1310 nm.

An effective area Δ_(eff) of at least one core region (in combinationswith its cladding regions) of the multicore glass fiber is greater than40 μm², or greater than 60 μm², or greater than 70 μm², or greater than80 μm², or in the range from 45 μm²-100 μm², or in the range from 55μm²-95 μm², or in the range from 60 μm²-90 μm², or in the range from 65μm²-85 μm² at a wavelength of 1550 nm.

A core spacing between the centerlines of at least one pair of adjacentcores in the multicore glass fiber is greater than 25 μm, or greaterthan 30 μm, or greater than 35 μm, or greater than 40 nm, or less than50 μm, or less than 45 μm, or in the range from 25 μm-50 μm, or in therange from 25 μm-40 μm, or in the range from 30 μm-45 μm, or in therange from 30 μm-40 μm, or in the range from 12 μm-28 μm, or in therange from 15 μm-25 μm. In some embodiments, a core spacing between thecenterlines of at least two pairs of adjacent cores in the multicoreglass fiber is greater than 25 μm, or greater than 30 μm, or greaterthan 35 μm, or greater than 40 μm, or less than 50 μm, or less than 45μm, or in the range from 25 μm-50 μm, or in the range from 25 μm-40 μm,or in the range from 30 μm-45 μm, or in the range from 30 μm-40 μm, orin the range from 12 μm-28 μm, or in the range from 15 μm-25 μm. In someembodiments, a core spacing between the centerlines of at least threepairs of adjacent cores in the multicore glass fiber is greater than 25μm, or greater than 30 μm, or greater than 35 μm, or greater than 40 μm,or less than 50 μm, or less than 45 μm, or in the range from 25 μm-50μm, or in the range from 25 μm-40 μm, or in the range from 30 μm-45 μm,or in the range from 30 μm-40 μm, or in the range from 12 μm-28 μm, orin the range from 15 μm-25 μm. In some embodiments, a core spacingbetween the centerlines of at least four pairs of adjacent cores in themulticore glass fiber is greater than 25 μm, or greater than 30 μm, orgreater than 35 μm, or greater than 40 μm, or less than 50 μm, or lessthan 45 μm, or in the range from 25 μm-50 μm, or in the range from 25μm-40 μm, or in the range from 30 μm-45 μm, or in the range from 30μm-40 μm, or in the range from 12 μm-28 μm, or in the range from 15μm-25 μm. In further embodiments, the core spacing or average corespacing of all pairs of adjacent cores is within the ranges stated inthis paragraph.

An edge spacing of at least one core region of the multicore opticalfiber is less than 30.0 μm, or less than 27.5 μm, or less than 25.0 μm,or less than 22.5 μm, or less than 20.0 μm, or in the range from 15.0μm-30.0 μm, or in the range from 17.5 μm-27.5 μm, or in the range from20.0 μm-25.0 μm. In some embodiments, an edge spacing of each of atleast two core regions of the multicore optical fiber is less than 30.0μm, or less than 27.5 μm, or less than 25.0 μm, or less than 22.5 μm, orless than 20.0 μm, or in the range from 15.0 μm-30.0 μm, or in the rangefrom 17.5 μm-27.5 μm, or in the range from 20.0 μm-25.0 μm. In otherembodiments, an edge spacing of each of at least three core regions ofthe multicore optical fiber is less than 30.0 μm, or less than 27.5 μm,or less than 25.0 μm, or less than 22.5 μm, or less than 20.0 μm, or inthe range from 15.0 μm-30.0 μm, or in the range from 17.5 μm-27.5 μm, orin the range from 20.0 μm-25.0 μm. In still other embodiments, an edgespacing of each of at least four core regions of the multicore opticalfiber is less than 30.0 μm, or less than 27.5 μm, or less than 25.0 μm,or less than 22.5 μm, or less than 20.0 μm, or in the range from 15.0μm-30.0 μm, or in the range from 17.5 μm-27.5 μm, or in the range from20.0 μm-25.0 μm. In still other embodiments, an edge spacing of each ofat least five core regions of the multicore optical fiber is less than30.0 μm, or less than 27.5 μm, or less than 25.0 μm, or less than 22.5μm, or less than 20.0 μm, or in the range from 15.0 μm-30.0 μm, or inthe range from 17.5 μm-27.5 μm, or in the range from 20.0 μm-25.0 μm. Infurther embodiments, the edge spacing or average edge spacing of allcore regions is within the ranges stated in this paragraph.

The degree of crosstalk at 1310 nm between at least one pair of adjacentcores in the multicore optical fiber is less than −40 dB/km, or lessthan −50 dB/km, or less than −60 dB/km, or in the range from −90 dB/kmto −40 dB/km, or in the range from −80 dB/km to −45 dB/km, or in therange from −70 dB/km to −50 dB/km. In some embodiments, the degree ofcrosstalk at 1310 nm between at least two pairs of adjacent cores in themulticore optical fiber is less than −40 dB/km, or less than −50 dB/km,or less than −60 dB/km, or in the range from −90 dB/km to −40 dB/km, orin the range from −80 dB/km to −45 dB/km, or in the range from −70 dB/kmto −50 dB/km. In other embodiments, the degree of crosstalk at 1310 nmbetween at least three pairs of adjacent cores in the multicore opticalfiber is less than −40 dB/km, or less than −50 dB/km, or less than −60dB/km, or in the range from −90 dB/km to −40 dB/km, or in the range from−80 dB/km to −45 dB/km, or in the range from −70 dB/km to −50 dB/km. Instill other embodiments, the degree of crosstalk at 1310 nm between atleast four pairs of adjacent cores in the multicore optical fiber isless than −40 dB/km, or less than −50 dB/km, or less than −60 dB/km, orin the range from −90 dB/km to −40 dB/km, or in the range from −80 dB/kmto −45 dB/km, or in the range from −70 dB/km to −50 dB/km. In furtherembodiments, the degree of crosstalk or average degree of crosstalk at1310 nm between all pairs of adjacent cores is within the ranges statedin this paragraph.

The degree of crosstalk at 1550 nm between at least one pair of adjacentcores in the multicore optical fiber is less than −30 dB/km, or lessthan −40 dB/km, or less than −50 dB/km, or in the range from −80 dB/kmto −30 dB/km, or in the range from −70 dB/km to −35 dB/km, or in therange from −60 dB/km to −40 dB/km. In some embodiments, the degree ofcrosstalk at 1550 nm between at least two pairs of adjacent cores in themulticore optical fiber is less than −30 dB/km, or less than −40 dB/km,or less than −50 dB/km, or in the range from −80 dB/km to −30 dB/km, orin the range from −70 dB/km to −35 dB/km, or in the range from −60 dB/kmto −40 dB/km. In other embodiments, the degree of crosstalk at 1550 nmbetween at least three pairs of adjacent cores in the multicore opticalfiber is less than −30 dB/km, or less than −40 dB/km, or less than −50dB/km, or in the range from −80 dB/km to −30 dB/km, or in the range from−70 dB/km to −35 dB/km, or in the range from −60 dB/km to −40 dB/km. Instill other embodiments, the degree of crosstalk at 1550 nm between atleast four pairs of adjacent cores in the multicore optical fiber isless than −30 dB/km, or less than −40 dB/km, or less than −50 dB/km, orin the range from −80 dB/km to −30 dB/km, or in the range from −70 dB/kmto −35 dB/km, or in the range from −60 dB/km to −40 dB/km. In furtherembodiments, the degree of crosstalk or average degree of crosstalk at1550 nm between all pairs of adjacent cores is within the ranges statedin this paragraph.

In one embodiment, a coating is applied to the outer surface of a commonouter cladding region. The coatings are formed from curable coatingcompositions. Curable coating compositions include one or more curablecomponents. As used herein, the term “curable” is intended to mean thatthe component, when exposed to a suitable source of curing energy,includes one or more curable functional groups capable of formingcovalent bonds that participate in linking the component to itself or toother components of the coating composition. The product obtained bycuring a curable coating composition is referred to herein as the curedproduct of the composition or as a coating. The cured product ispreferably a polymer. The curing process is induced by energy. Forms ofenergy include electromagnetic radiation or thermal energy.

A curable component includes one or more curable functional groups. Acurable component with only one curable functional group is referred toherein as a monofunctional curable component. A curable component havingtwo or more curable functional groups is referred to herein as amultifunctional curable component. Multifunctional curable componentscan introduce crosslinks into the polymeric network that forms duringthe curing process. Multifunctional curable components may also bereferred to herein as “crosslinkers” or “curable crosslinkers”. Curablecomponents include curable monomers and curable oligomers. Examples offunctional groups that participate in covalent bond formation during thecuring process are acrylate groups and methacrylate groups.

The coating preferably includes a primary coating surrounding anddirectly adjacent to a common outer cladding region and a secondarycoating surrounding and directly adjacent to the primary coating. Thesecondary coating is a harder material (higher Young's modulus) than theprimary coating and is designed to protect the multicore glass fiberfrom damage caused by abrasion or external forces that arise duringprocessing, handling, and installation of the multicore optical fiber.The primary coating is a softer material (lower Young's modulus) thanthe secondary coating and is designed to buffer or dissipates stressesthat result from forces applied to the outer surface of the secondarycoating. Dissipation of stresses within the primary coating attenuatesthe stress and minimizes the stress that reaches the multicore glassfiber. The primary coating is especially important in dissipatingstresses that arise when the multicore optical fiber is bent. Themulticore optical fiber may also include a tertiary coating thatsurrounds and is directly adjacent to the secondary coating. Thetertiary coating may include pigments, inks or other coloring agents tomark the optical fiber for identification purposes and typically has aYoung's modulus similar to the Young's modulus of the secondary coating.

Primary and secondary coatings are typically formed on the draw byapplying a curable coating composition to the multicore glass fiber as aviscous liquid and curing. In a continuous optical fiber manufacturingprocess, a glass fiber is drawn from a heated preform and sized to atarget diameter. The glass fiber is then cooled and directed to acoating system that applies a liquid primary coating composition to theglass fiber. Two process options are viable after application of theliquid primary coating composition to the glass fiber. In one processoption (wet-on-dry process), the liquid primary coating composition iscured to form a solidified primary coating, the liquid secondary coatingcomposition is applied to the cured primary coating, and the liquidsecondary coating composition is cured to form a solidified secondarycoating. In a second process option (wet-on-wet process), the liquidsecondary coating composition is applied to the liquid primary coatingcomposition, and both liquid coating compositions are curedsimultaneously to provide solidified primary and secondary coatings.After the fiber exits the coating system, the fiber is collected andstored at room temperature. Collection of the fiber typically entailswinding the fiber on a spool and storing the spool.

The primary coating is a cured product of a radiation-curable primarycoating composition that includes an oligomer, a monomer, aphotoinitiator and, optionally, an additive.

The oligomer preferably includes a polyether urethane diacrylatecompound and a di-adduct compound. In one embodiment, the polyetherurethane diacrylate compound has a linear molecular structure. In oneembodiment, the oligomer is formed from a reaction between adiisocyanate compound, a polyol compound, and a hydroxy acrylatecompound, where the reaction produces a polyether urethane diacrylatecompound as a primary product (majority product) and a di-adductcompound as a byproduct (minority product). The reaction forms aurethane linkage upon reaction of an isocyanate group of thediisocyanate compound and an alcohol group of the polyol. The hydroxyacrylate compound reacts to quench residual isocyanate groups that arepresent in the composition formed from reaction of the diisocyanatecompound and polyol compound. As used herein, the term “quench” refersto conversion of isocyanate groups through a chemical reaction withhydroxyl groups of the hydroxy acrylate compound. Quenching of residualisocyanate groups with a hydroxy acrylate compound converts terminalisocyanate groups to terminal acrylate groups. The di-adduct compound isa diacrylate compound formed by reaction of both isocyanate groups ofthe diisocyanate compound with the hydroxy acrylate compound.

The one or more monomers is/are selected to be compatible with theoligomer, to control the viscosity of the primary coating composition tofacilitate processing, and/or to influence the physical or chemicalproperties of the coating formed as the cured product of the primarycoating composition. The monomers include radiation-curable monomerssuch as ethylenically-unsaturated compounds, ethoxylated acrylates,ethoxylated alkylphenol monoacrylates, propylene oxide acrylates,n-propylene oxide acrylates, isopropylene oxide acrylates,monofunctional acrylates, monofunctional aliphatic epoxy acrylates,multifunctional acrylates, multifunctional aliphatic epoxy acrylates,and combinations thereof.

Representative radiation-curable ethylenically unsaturated monomersinclude alkoxylated monomers with one or more acrylate or methacrylategroups. An alkoxylated monomer is one that includes one or morealkoxylene groups, where an alkoxylene group has the form —O—R— and R isa linear or branched alkylene group. Examples of alkoxylene groupsinclude ethoxylene (—O—CH₂—CH₂—), n-propoxylene (—O—CH₂—CH₂—CH₂—),isopropoxylene (—O—CH₂—CH(CH₃)—, or —O—CH(CH₃)—CH₂—), etc. In someembodiments, the primary coating composition includes an alkoxylatedmonomer of the form R₄—R₅—O—(CH(CH₃)CH₂—O)_(q)—C(O)CH═CH₂, where R₄ andR₅ are aliphatic, aromatic, or a mixture of both, and q=1 to 10, orR₄—O—(CH(CH₃)CH₂—O)_(q)—C(O)CH═CH₂, where C(O) is a carbonyl group, R₁is aliphatic or aromatic, and q=1 to 10.

Representative examples of monofunctional monomers include ethylenicallyunsaturated monomers such as lauryl acrylate, ethoxylated nonylphenolacrylate, caprolactone acrylate, phenoxyethyl acrylate, isooctylacrylate, tridecyl acrylate, isobornyl acrylate, tetrahydrofurfurylacrylate, stearyl acrylate, isodecyl acrylate, 2-(2-ethoxyethoxy)ethylacrylate, epoxy acrylate, lauryloxyglycidyl acrylate and phenoxyglycidylacrylate and combinations thereof. Examples of multifunctional monomersinclude dipentaerythritol monohydroxy pentaacrylate, methylolpropanepolyacrylates with and without alkoxylation such as trimethylolpropanetriacrylate and ditrimethylolpropane tetraacrylate, alkoxylated glyceryltriacrylates such as propoxylated glyceryl triacrylate withpropoxylation being 3 or greater, and erythritol polyacrylates with andwithout alkoxylation, such as pentaerythritol tetraacrylate, ethoxylatedpentaerythritol tetraacrylate, dipentaerythritol pentaacrylate,tripropyleneglycol diacrylate, propoxylated hexanediol diacrylate,tetrapropyleneglycol diacrylate, pentapropyleneglycol diacrylate,methacrylate analogs of the foregoing, and combinations thereof. Othermonomers include N-vinyl amide monomers such as an N-vinyl lactam, orN-vinyl pyrrolidinone, or N-vinyl caprolactam.

The photoinitiator facilitates initiation of the polymerization processassociated with the curing of the primary coating composition to formthe coating. Photoinitiators include ketonic photoinitiators and/orphosphine oxide photoinitiators. When used in the curing of the coatingcomposition, the photoinitiator is present in an amount sufficient toenable rapid radiation curing. Representative photoinitiators include1-hydroxycyclohexylphenyl ketone;bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide;2,2-dimethoxy-2-phenylacetophenone;bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide;(2,4,6-trimethylbenzoyl)diphenyl phosphine oxide;ethoxy(2,4,6-trimethylbenzoyl)-phenylphosphine oxide; and combinationsthereof.

The curable primary coating composition optionally includes one or moreadditives. Additives include an adhesion promoter, a strength additive,an antioxidant, a catalyst, a stabilizer, an optical brightener, aproperty-enhancing additive, an amine synergist, a wax, a lubricant,and/or a slip agent. Some additives operate to control thepolymerization process, thereby affecting the physical properties (e.g.,modulus, glass transition temperature) of the polymerization productformed from the coating composition. Other additives affect theintegrity of the cured product of the primary coating composition (e.g.,protect against de-polymerization or oxidative degradation.

The secondary coating is a cured product of a curable secondary coatingcomposition that includes a monomer, a photoinitiator, an optionaloligomer, and an optional additive.

The monomers preferably include ethylenically unsaturated compounds. Theone or more monomers may be present in an amount of 50 wt % or greater,or in an amount from about 60 wt % to about 99 wt %, or in an amountfrom about 75 wt % to about 99 wt %, or in an amount from about 80 wt %to about 99 wt % or in an amount from about 85 wt % to about 99 wt %. Inone embodiment, the secondary coating is the radiation-cured product ofa secondary coating composition that contains urethane acrylatemonomers.

Exemplary monofunctional ethylenically unsaturated monomers for thecurable secondary coating composition include, without limitation,hydroxyalkyl acrylates such as 2-hydroxyethyl-acrylate,2-hydroxypropyl-acrylate, and 2-hydroxybutyl-acrylate; long- andshort-chain alkyl acrylates such as methyl acrylate, ethyl acrylate,propyl acrylate, isopropyl acrylate, butyl acrylate, amyl acrylate,isobutyl acrylate, t-butyl acrylate, pentyl acrylate, isoamyl acrylate,hexyl acrylate, heptyl acrylate, octyl acrylate, isooctyl acrylate,2-ethylhexyl acrylate, nonyl acrylate, decyl acrylate, isodecylacrylate, undecyl acrylate, dodecyl acrylate, lauryl acrylate, octadecylacrylate, and stearyl acrylate; aminoalkyl acrylates such asdimethylaminoethyl acrylate, diethylaminoethyl acrylate, and7-amino-3,7-dimethyloctyl acrylate; alkoxyalkyl acrylates such asbutoxyethyl acrylate, phenoxyethyl acrylate, and ethoxyethoxyethylacrylate; single and multi-ring cyclic aromatic or non-aromaticacrylates such as cyclohexyl acrylate, benzyl acrylate,dicyclopentadiene acrylate, dicyclopentanyl acrylate, tricyclodecanylacrylate, bornyl acrylate, isobornyl acrylate, tetrahydrofurfurylacrylate, caprolactone acrylate, and acryloylmorpholine; alcohol-basedacrylates such as polyethylene glycol monoacrylate, polypropylene glycolmonoacrylate, methoxyethylene glycol acrylate, methoxypolypropyleneglycol acrylate, methoxypolyethylene glycol acrylate, ethoxydiethyleneglycol acrylate, and various alkoxylated alkylphenol acrylates such asethoxylated(4) nonylphenol acrylate; acrylamides such as diacetoneacrylamide, isobutoxymethyl acrylamide, N,N′-dimethyl-aminopropylacrylamide, N,N-dimethyl acrylamide, N,N diethyl acrylamide, and t-octylacrylamide; vinylic compounds such as N-vinylpyrrolidone andN-vinylcaprolactam; and acid esters such as maleic acid ester andfumaric acid ester. With respect to the long and short chain alkylacrylates listed above, a short chain alkyl acrylate is an alkyl groupwith 6 or less carbons and a long chain alkyl acrylate is an alkyl groupwith 7 or more carbons.

Representative radiation-curable ethylenically unsaturated monomers forthe curable secondary composition include alkoxylated monomers with oneor more acrylate or methacrylate groups. An alkoxylated monomer is onethat includes one or more alkoxylene groups, where an alkoxylene grouphas the form —O—R— and R is a linear or branched hydrocarbon. Examplesof alkoxylene groups include ethoxylene (—O—CH₂—CH₂—), n-propoxylene(—O—CH₂—CH₂—CH₂—), isopropoxylene (—O—CH₂—CH(CH₃)—), etc.

Representative multifunctional ethylenically unsaturated monomers forthe curable secondary coating composition include, without limitation,alkoxylated bisphenol A diacrylates, such as ethoxylated bisphenol Adiacrylate, with the degree of alkoxylation being 2 or greater. Themonomer component of the secondary coating composition may includeethoxylated bisphenol A diacrylate with a degree of ethoxylation rangingfrom 2 to about 30, or propoxylated bisphenol A diacrylate with thedegree of propoxylation being 2 or greater; for example, ranging from 2to about 30; methylolpropane polyacrylates with and without alkoxylationsuch as ethoxylated trimethylolpropane triacrylate with the degree ofethoxylation being 3 or greater; for example, ranging from 3 to about30; propoxylated-trimethylolpropane triacrylate with the degree ofpropoxylation being 3 or greater; for example, ranging from 3 to 30;ditrimethylolpropane tetraacrylate; alkoxylated glyceryl triacrylatessuch as propoxylated glyceryl triacrylate with the degree ofpropoxylation being 3 or greater; erythritol polyacrylates with andwithout alkoxylation, such as pentaerythritol tetraacrylate, ethoxylatedpentaerythritol tetraacrylate, and dipentaerythritol pentaacrylate;isocyanurate polyacrylates formed by reacting an appropriate functionalisocyanurate with an acrylic acid or acryloyl chloride, such astris-(2-hydroxyethyl) isocyanurate triacrylate and tris-(2-hydroxyethyl)isocyanurate diacrylate; alcohol polyacrylates with and withoutalkoxylation such as tricyclodecane dimethanol diacrylate andethoxylated polyethylene glycol diacrylate with the degree ofethoxylation being 2 or greater; for example, ranging from about 2 to30; epoxy acrylates formed by adding acrylate to bisphenol Adiglycidylether and the like; and single and multi-ring cyclic aromaticor non-aromatic polyacrylates such as dicyclopentadiene diacrylate anddicyclopentane diacrylate.

The optional oligomer present in the radiation-curable secondary coatingcomposition is preferably a compound with urethane linkages. In oneaspect, the optional oligomer is a reaction product of a polyolcompound, a diisocyanate compound, and a hydroxy-functional acrylatecompound. Reaction of the polyol compound with the diisocyanate compoundprovides a urethane linkage and the hydroxy-functional acrylate compoundreacts with isocyanate groups to provide terminal acrylate groups.

The curable secondary coating composition also includes a photoinitiatorand optionally includes additives such as an anti-oxidant, an opticalbrightener, an amine synergist, a tackifier, a catalyst, a carrier orsurfactant, and a stabilizer as described above in connection with thecurable primary coating composition.

Fiber Draw Process. In a continuous optical fiber manufacturing process,a glass fiber is drawn from a heated preform and sized to a targetdiameter (typically 125 μm). The glass fiber is then cooled at acontrolled rate and directed to a coating system that applies a liquidprimary coating composition to the glass fiber. Two process options areviable after application of the liquid primary coating composition tothe glass fiber. In one process option (wet-on-dry process), the liquidprimary coating composition is cured to form a solidified primarycoating, the liquid secondary coating composition is applied to thecured primary coating, and the liquid secondary coating composition iscured to form a solidified secondary coating. In a second process option(wet-on-wet process), the liquid secondary coating composition isapplied to the liquid primary coating composition, and both liquidcoating compositions are cured simultaneously to provide solidifiedprimary and secondary coatings. After the fiber exits the coatingsystem, the fiber is collected and stored at room temperature.Collection of the fiber typically entails winding the fiber on a spooland storing the spool. Fluid bearing devices may be used to turn andredirect the optical fiber during the draw process.

In some processes, the coating system further applies a tertiary coatingcomposition to the secondary coating and cures the tertiary coatingcomposition to form a solidified tertiary coating. Typically, thetertiary coating is an ink layer used to mark the fiber foridentification purposes and has a composition that includes a pigmentand is otherwise similar to the secondary coating. The tertiary coatingis applied to the secondary coating and cured. The secondary coating hastypically been cured at the time of application of the tertiary coating.The primary, secondary, and tertiary coating compositions can be appliedand cured in a common continuous manufacturing process. Alternatively,the primary and secondary coating compositions are applied and cured ina common continuous manufacturing process, the coated fiber iscollected, and the tertiary coating composition is applied and cured ina separate offline process to form the tertiary coating.

The wavelength of curing radiation is infrared, visible, or ultraviolet(UV). Representative wavelengths include wavelengths in the range from250 nm to 1000 nm, or in the range from 250 nm to 700 nm, or in therange from 250 nm to 450 nm, or in the range from 275 nm to 425 nm, orin the range from 300 nm to 400 nm, or in the range from 320 nm to 390nm, or in the range from 330 nm to 380 nm, or in the range from 340 nmto 370 nm. Curing can be accomplished with light sources that include alamp source (e.g. Hg lamp), an LED source (e.g. a UVLED, visible LED, orinfrared LED), or a laser source.

To improve process efficiency, it is desirable to increase the drawspeed of the fiber along the process pathway extending from the preformto the collection point. As the draw speed increases, however, the curespeed of coating compositions must increase. The coating compositionsdisclosed herein are compatible with fiber draw processes that operateat a draw speed greater than 35 m/s, or greater than 40 m/s, or greaterthan 45 m/s, or greater than 50 m/s, or greater than 55 m/s, or greaterthan 60 m/s, or greater than 65 m/s, or greater than 70 m/s.

Examples

The examples that follow present calculated results that illustrate areduction in tunneling loss associated with a common exterior claddingregion as described herein as well as model designs of multicore opticalfibers that include various numbers and geometric arrangement of coresin a multicore optical fiber.

Tunneling Loss. To illustrate the benefit of including a common exteriorcladding region that surrounds and is directly adjacent to a commoninterior cladding region, the tunneling loss as a function of edgespacing was calculated for cores having two different relativerefractive index profiles—Profile 1 and Profile 2. The parameter valuesassociated with each profile are listed in Table 1. Both profiles arestep index profiles. Profile 1 has a profile of the type shown in FIG. 5and Profile 2 has a profile of the type shown in FIG. 2. Each profileincluded a common exterior cladding region with a radius R₅=62.5 μm anda relative refractive index Δ₄=0.0%.

TABLE 1 Parameter Profile 1 Profile 2 Δ₁ (%) 0.34 0.34 α 200 200 r₁ (μm)4.2 4.2 Δ₂ (%) −0.40 0 r₂ (μm) 9.2 10.2 Δ₃ (%) — −0.40 r₃ (μm) — 15.2

Calculations of tunneling loss at a wavelength of 1550 nm were made as afunction of edge spacing for cores having Profile 1 and Profile 2 listedin Table 1. The calculation was based on a single core with eachprofile. For each profile, calculations were made for threeconfigurations: a first configuration having a common interior claddingregion (with radius R₄=62.5 μm) and no common exterior cladding region,a second configuration having a common interior cladding region and acommon exterior cladding region having thickness R₅−R₄=5 μm and relativerefractive index Δ₅=−0.40%, and a third configuration having a commoninterior cladding region and a common exterior cladding region havingthickness R₅−R₄=10 μm and relative refractive index Δ₅=−0.40%.

FIGS. 11 and 12 show the calculated tunneling loss for Profile 1 andProfile 2, respectively. Traces 82, 84, and 86 of FIG. 11 show thetunneling loss for the first, second, and third configurations notedabove for Profile 1. Traces 92, 94, and 96 of FIG. 12 show the tunnelingloss for the first, second, and third configurations noted above forProfile 2. Traces 82 and 92 show that the tunneling loss at a given edgespacing is greatest when no common exterior cladding is present.Inclusion of a common exterior cladding with a thickness of 5 μm led toa significant reduction in tunneling loss (Traces 84 and 94). An evengreater reduction in tunneling loss is observed when the thickness ofthe common exterior cladding is increased to 10 μm (Traces 86 and 96).

An alternative way of interpreting the results shown in FIGS. 11 and 12is in terms of the edge spacing required to reduce the tunneling loss toa specified level. In FIG. 11, for example, a tunneling loss of 0.1dB/km requires an edge spacing of about 34 μm when no common exteriorcladding is present (Trace 82), an edge spacing of about 27 μm when acommon exterior cladding with a thickness of 5 μm is present (Trace 84),and an edge spacing of slightly greater than 20 μm when a commonexterior cladding with a thickness of 10 μm is present (Trace 86). InFIG. 12, a tunneling loss of 0.04 dB/km requires an edge spacing ofabout 27.5 μm when no common exterior cladding is present (Trace 92), anedge spacing of about 23.5 μm when a common exterior cladding with athickness of 5 μm is present (Trace 94), and an edge spacing of slightlygreater than 20 μm when a common exterior cladding with a thickness of10 μm is present (Trace 96). The results show that cores can bepositioned closer to the edge of a multicore optical fiber when a commonexterior cladding as described herein is included. As a result, a largernumber of cores can be included in multicore fibers of a given diameterwithout increasing losses due to tunneling when including a commonexterior cladding as described herein.

Model Designs. FIGS. 13-17 show illustrative designs of multicoreoptical fibers consistent with the present disclosure. Each design isillustrated with a cross-sectional view and includes multiple cores(depicted as circles with small radius) surrounded by a common interiorcladding (depicted as circles with intermediate radius), which issurrounded by a common exterior cladding (depicted as circles with largeradius). Table 2 shows geometric parameters and figure number for eachdesign. Table 3 lists relative refractive indices, and radii of thedifferent regions of each design. Table 4 lists selected opticalproperties for each design. Each core in each design has a step indexprofile of the type shown in FIG. 2. Within a design, each core had thesame values of α, r₁, r₂, r₃, Δ₁, Δ₂, and Δ₃. The optical propertieslisted in Table 4 correspond to optical properties of each individualcore of each design. The listing “na” means not applicable.

TABLE 2 Design 1 Design 2 Design 3 Design 4 Design 5 Design 6 FIG 13 1414 15 16 17 # of Cores 8 4 4 8 7 37 Core Ring 1 × 4 1 × 4 2 × 4Hexagonal Hexagonal Geometry linear linear linear Core 33 30 30 28 40 35Spacing (μm) Edge 20 17.5 17.5 18.2 22.5 20 Spacing (μm) Core 125 125125 125 125 250 Diameter (μm)

TABLE 3 Design 1 Design 2 Design 3 Design 4 Design 5 Design 6 Δ₁ (%)0.35 0.35 0.5 0.5 0.36 0.4 r₁ (μm) 3.25 3.25 3.6 3.6 4.9 4.6 α 200 20020 20 200 20 Δ₂ (%) 0.05 0.05 0 0 0 0 r₂ (μm) 7.95 7.95 6 6 7.75 7 Δ₃(%) −0.698 −0.698 −0.40 −0.40 −0.53 −0.40 r₃ (μm) 12.88 12.88 12 12 1513 Δ₄ (%) 0 0 0 0 0 0 R₄ (μm) 55.5 58.0 57.0 56.5 52.5 52.5 Δ₅ (%) −0.7−0.7 −1.0 −1.0 −0.5 −0.5 R₅ (μm) 62.5 62.5 62.5 62.5 62.5 125

TABLE 4 Design 1 Design 2 Design 3 Design 4 Design 5 Design 6 λ_(CC)(nm) 1240 1240 1220 1220 1500 1500 MFD-1310 nm (μm) 8.6 8.6 7.2 7.2 nana A_(eff)-1310 nm (μm²) 56.6 56.6 42.1 42.1 na na Dispersion at 1310−0.19 −0.19 1.5 1.5 na na nm (ps/nm − km) Dispersion Slope at 0.09750.0975 0.0907 0.0907 na na 1310 nm (ps/nm² − km) Crosstalk at 1310 nm−85 −68 −83 −72 na na (dB/km) MFD-1550 nm (μm) 9.6 9.6 8.0 8.0 9.8 9.3A_(eff)-1550 nm (μm²) 70.7 70.7 49.8 49.8 80.0 70.0 Dispersion at 155019.3 19.3 19.2 19.2 22.4 21.1 nm (ps/nm − km) Dispersion Slope at 0.06940.0694 0.0617 0.0617 0.0645 0.0624 1550 nm (ps/nm² − km) Crosstalk at1550 nm −62 −48 −60 −50 −80 −71 (dB/km)

The modelling results indicate that the tunneling decreasesapproximately exponentially with increasing profile area of the commonexterior cladding region. The profile area, A_(P), is defined in Eq.(9):A _(P)=|Δ₅(R ₅ −R ₄)|  (9)where Δ₅ is the relative refractive index of the common exteriorcladding region and the difference R₅−R₄ is the thickness of the commonexterior cladding region. The profile area is expressed in units of % μmand is given in Table 5 for each of the illustrative designs consideredin this example.

TABLE 5 Design 1 Design 2 Design 3 Design 4 Design 5 Design 6 Δ₅ (%)−0.7 −0.7 −1 −1 −0.5 −0.5 R₅ − R₄ (μm) 7.0 4.5 5.5 6.0 10 10 A_(P) (%μm) 4.9 3.15 5.5 6.0 5.0 5.0

For low tunneling loss, the profile area A_(P) is greater than or equalto 2.0% μm, or greater than or equal to 3.0% μm, or greater than orequal to 4.0% μm, or greater than or equal to 5.0% μm, or greater thanor equal to 6.0% μm, or in the range from 2.0% μm to 10.0% μm, or in therange from 3.0% μm to 9.0% μm, or in the range from 4.0% μm to 8.0% μm.

Aspect 1 of the description is:

A multicore optical fiber comprising:

-   -   a multicore glass fiber, the multicore glass fiber comprising:        -   a plurality of core regions, each of the plurality of core            regions surrounded by and directly adjacent to a dedicated            inner cladding region;        -   a common interior cladding region surrounding the dedicated            inner cladding region of each of the plurality of core            regions, the common interior cladding region having a            relative refractive index Δ₄ and a radius R₄; and    -   a common exterior cladding region surrounding the common        interior cladding region, the common exterior cladding region        having a radius R₅>R₄ and a relative refractive index Δ₅<Δ₄.

Aspect 2 of the description is:

The multicore optical fiber of Aspect 1, wherein the plurality of coreregions includes 4 or more core regions.

Aspect 3 of the description is:

The multicore optical fiber of Aspect 1, wherein the plurality of coreregions includes 8 or more core regions.

Aspect 4 of the description is:

The multicore optical fiber of any of Aspects 1-3, wherein each of theplurality of core regions has a radius r₁ in the range from 3.0 μm-8.0μm and a relative refractive index Δ₁ in the range from 0.20%-0.80%.

Aspect 5 of the description is:

The multicore optical fiber of any of Aspects 1-4, wherein the dedicatedinner cladding region of each of the plurality of core regions has aradius r₂ in the range from 9.0 μm-18.0 μm and a relative refractiveindex Δ₂ in the range from −0.80% to −0.10%.

Aspect 6 of the description is:

The multicore optical fiber of Aspect 5, wherein the common interiorcladding region is directly adjacent to the dedicated inner claddingregion of each of the plurality of core regions.

Aspect 7 of the description is:

The multicore optical fiber of any of Aspects 1-4, wherein the dedicatedinner cladding region of each of the plurality of core regions has aradius r₂ in the range from 4.0 μm-10.0 μm and a relative refractiveindex Δ₂ in the range from −0.10% to 0.10%.

Aspect 8 of the description is:

The multicore optical fiber of Aspect 7, wherein each of the pluralityof core regions is further surrounded by a dedicated depressed indexcladding region directly adjacent to the dedicated inner claddingregion, the dedicated depressed index cladding region having a radius r₃in the range from the range from 7.0 μm-20.0 μm and a relativerefractive index Δ₃ in the range from −0.80% to −0.10%.

Aspect 9 of the description is:

The multicore optical fiber of Aspect 8, wherein the common interiorcladding region is directly adjacent to the dedicated depressed indexcladding region of each of the plurality of core regions.

Aspect 10 of the description is:

The multicore optical fiber of any of Aspects 1-9, wherein the relativerefractive index Δ₄ is in the range from −0.30% to 0.30% and the radiusR₄ is in the range from 50.0 μm-125.0 μm.

Aspect 11 of the description is:

The multicore optical fiber of any of Aspects 1-9, wherein the relativerefractive index Δ₄ is in the range from −0.20% to 0.20% and the radiusR₄ is in the range from 50.0 μm-60.0 μm.

Aspect 12 of the description is:

The multicore optical fiber of any of Aspects 1-11, wherein the relativerefractive index Δ₅ is in the range from −0.80% to −0.10%.

Aspect 13 of the description is:

The multicore optical fiber of any of Aspects 1-11, wherein the relativerefractive index Δ₅ is less than −0.30%.

Aspect 14 of the description is:

The multicore optical fiber of any of Aspects 1-13, wherein a differenceΔ₄−Δ₅ is greater than 0.10%.

Aspect 15 of the description is:

The multicore optical fiber of any of Aspects 1-13, wherein a differenceΔ₄−Δ₅ is greater than 0.30%.

Aspect 16 of the description is:

The multicore optical fiber of any of Aspects 1-15, wherein the radiusR₅ is in the range from 50.0 μm-130.0 μm.

Aspect 17 of the description is:

The multicore optical fiber of any of Aspects 1-15, wherein the radiusR₅ is in the range from 58.0 μm-63.0 μm.

Aspect 18 of the description is:

The multicore optical fiber of any of Aspects 1-17, wherein a differenceR₅−R₄ is in the range from 1.0 μm-30.0 μm.

Aspect 19 of the description is:

The multicore optical fiber of any of Aspects 1-17, wherein a differenceR₅−R₄ is in the range from 4.0 μm-12.0 μm.

Aspect 20 of the description is:

The multicore optical fiber of any of Aspects 1-19, wherein the commonexterior cladding region has a profile area A_(P) greater than or equalto 2.0% μm.

Aspect 21 of the description is:

The multicore optical fiber of any of Aspects 1-20, wherein the commonexterior cladding region is directly adjacent to the common interiorcladding region.

Aspect 22 of the description is:

The multicore optical fiber of any of Aspects 1-21, wherein each of theplurality of core regions has a mode field diameter (MFD) at 1310 nmgreater than 7.0 μm.

Aspect 23 of the description is:

The multicore optical fiber of any of Aspects 1-21, wherein each of theplurality of core regions has a mode field diameter (MFD) at 1550 nmgreater than 8.0 μm.

Aspect 24 of the description is:

The multicore optical fiber of any of Aspects 1-23, wherein theplurality of core regions comprises at least one pair of adjacent coreshaving a core spacing greater than 25 μm.

Aspect 25 of the description is:

The multicore optical fiber of any of Aspects 1-23, wherein theplurality of core regions comprises at least one pair of adjacent coreshaving a core spacing greater than 30 μm.

Aspect 26 of the description is:

The multicore optical fiber of any of Aspects 1-25, wherein an averagecore spacing of all pairs of adjacent core regions of the plurality ofcore regions is greater than 25 μm.

Aspect 27 of the description is:

The multicore optical fiber of any of Aspects 1-25, wherein an averagecore spacing of all pairs of adjacent core regions of the plurality ofcore regions is greater than 30 μm.

Aspect 28 of the description is:

The multicore optical fiber of any of Aspects 1-27, wherein theplurality of core regions comprises at least one core region having anedge spacing less than 30.0 μm.

Aspect 29 of the description is:

The multicore optical fiber of any of Aspects 1-27, wherein theplurality of core regions comprises at least one core region having anedge spacing less than 25.0 μm.

Aspect 30 of the description is:

The multicore optical fiber of any of Aspects 1-27, wherein theplurality of core regions comprises at least one core region having anedge spacing less than 20.0 μm.

Aspect 31 of the description is:

The multicore optical fiber of any of Aspects 1-30, wherein theplurality of core regions comprises at least two core regions having anedge spacing less than 30.0 μm.

Aspect 32 of the description is:

The multicore optical fiber of any of Aspects 1-30, wherein theplurality of core regions comprises at least two core regions having anedge spacing less than 25.0 μm.

Aspect 33 of the description is:

The multicore optical fiber of any of Aspects 1-30, wherein theplurality of core regions comprises at least two core regions having anedge spacing less than 20.0 μm.

Aspect 34 of the description is:

The multicore optical fiber of any of Aspects 1-33, wherein theplurality of core regions comprises at least three core regions havingan edge spacing less than 30.0 μm.

Aspect 35 of the description is:

The multicore optical fiber of any of Aspects 1-33, wherein theplurality of core regions comprises at least three core regions havingan edge spacing less than 25.0 μm.

Aspect 36 of the description is:

The multicore optical fiber of any of Aspects 1-33, wherein theplurality of core regions comprises at least three core regions havingan edge spacing less than 20.0 μm.

Aspect 37 of the description is:

The multicore optical fiber of any of Aspects 1-36, wherein a crosstalkbetween each pair of adjacent core regions of the plurality of coreregions is less than −60 dB/km at 1310 nm.

Aspect 38 of the description is:

The multicore optical fiber of any of Aspects 1-36, wherein a crosstalkbetween each pair of adjacent core regions of the plurality of coreregions is less than −80 dB/km at 1310 nm.

Aspect 39 of the description is:

The multicore optical fiber of any of Aspects 1-36, wherein a crosstalkbetween each pair of adjacent core regions of the plurality of coreregions is less than −50 dB/km at 1550 nm.

Aspect 40 of the description is:

The multicore optical fiber of any of Aspects 1-36, wherein a crosstalkbetween each pair of adjacent core regions of the plurality of coreregions is less than −70 dB/km at 1550 nm.

Aspect 41 of the description is:

The multicore optical fiber of any of Aspects 1-40, further comprising acoating surrounding and directly adjacent to the common exteriorcladding region, the coating comprising a polymer.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is no way intended thatany particular order be inferred.

It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thespirit or scope of the invention. Since modifications combinations,sub-combinations and variations of the disclosed embodimentsincorporating the spirit and substance of the invention may occur topersons skilled in the art, the invention should be construed to includeeverything within the scope of the appended claims and theirequivalents.

What is claimed is:
 1. A multicore optical fiber comprising: a multicoreglass fiber, the multicore glass fiber comprising: a plurality of coreregions, each of the plurality of core regions surrounded by anddirectly adjacent to a dedicated inner cladding region, the dedicatedinner cladding region having a radius r₂ in the range from 4.0 μm-10.0μm and a relative refractive index Δ₂ in the range from −0.10% to 0.10%,each of the plurality of core regions further surrounded by a dedicateddepressed index cladding region directly adjacent to the dedicated innercladding region, the dedicated depressed index cladding region having aradius r₃ in the range from the range from 7.0 μm-20.0 μm and a relativerefractive index Δ₃ in the range from −0.80% to −0.10%; a commoninterior cladding region surrounding the dedicated depressed indexcladding region of each of the plurality of core regions, the commoninterior cladding region having a relative refractive index Δ₄ and aradius R₄; and a common exterior cladding region surrounding the commoninterior cladding region, the common exterior cladding region having aradius R₅>R₄ and a relative refractive index Δ₅<Δ₄.
 2. The multicoreoptical fiber of claim 1, wherein the plurality of core regions includes4 or more core regions.
 3. The multicore optical fiber of claim 1,wherein each of the plurality of core regions has a radius r₁ in therange from 3.0 μm-8.0 μm and a relative refractive index Δ₁ in the rangefrom 0.20%-0.80%.
 4. The multicore optical fiber of claim 1, wherein thededicated inner cladding region of each of the plurality of core regionshas a radius r₂ in the range from 9.0 μm-18.0 μm and a relativerefractive index Δ₂ in the range from −0.80% to −0.10%.
 5. The multicoreoptical fiber of claim 4, wherein the common interior cladding region isdirectly adjacent to the dedicated inner cladding region of each of theplurality of core regions.
 6. The multicore optical fiber of claim 1,wherein the common interior cladding region is directly adjacent to thededicated depressed index cladding region of each of the plurality ofcore regions.
 7. The multicore optical fiber of claim 1, wherein therelative refractive index Δ₄ is in the range from −0.30% to 0.30% andthe radius R₄ is in the range from 50.0 μm-125.0 μm.
 8. The multicoreoptical fiber of claim 1, wherein the relative refractive index Δ₅ is inthe range from −0.80% to −0.10%.
 9. The multicore optical fiber of claim1, wherein a difference Δ₄−Δ₅ is greater than 0.10%.
 10. The multicoreoptical fiber of claim 1, wherein the radius R₅ is in the range from50.0 μm-130.0 μm.
 11. The multicore optical fiber of claim 1, wherein adifference R₅−R₄ is in the range from 4.0 μm-12.0 μm.
 12. The multicoreoptical fiber of claim 1, wherein the common exterior cladding regionhas a profile area A_(P) greater than or equal to 2.0% μm.
 13. Themulticore optical fiber of claim 1, wherein the common exterior claddingregion is directly adjacent to the common interior cladding region. 14.The multicore optical fiber of claim 1, wherein each of the plurality ofcore regions has a mode field diameter (MFD) at 1310 nm greater than 7.0μm.
 15. The multicore optical fiber of claim 1, wherein an average corespacing of all pairs of adjacent core regions of the plurality of coreregions is greater than 25 μm.
 16. The multicore optical fiber of claim1, wherein the plurality of core regions comprises at least one coreregion having an edge spacing less than 20.0 μm.
 17. The multicoreoptical fiber of claim 1, wherein the plurality of core regionscomprises at least two core regions having an edge spacing less than30.0 μm.
 18. The multicore optical fiber of claim 1, wherein theplurality of core regions comprises at least three core regions havingan edge spacing less than 30.0 μm.
 19. The multicore optical fiber ofclaim 1, wherein a crosstalk between each pair of adjacent core regionsof the plurality of core regions is less than −60 dB/km at 1310 nm. 20.The multicore optical fiber of claim 1, wherein a crosstalk between eachpair of adjacent core regions of the plurality of core regions is lessthan −50 dB/km at 1550 nm.